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Old 1-Jun-2020, 4:34 AM   #1
bobsgarage
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91 XG Modified

So I made lemonade out of lemons today. I ordered a couple of 91XGs through Amazon last week. I was excited when I got home from work, tore the Box open and found at the UPS handling system destroyed part of the antennas. The booms were bent. When I called Amazon to get the antennas replaced they said they would send two more and I could keep the bent ones.

I took that smashed up boom from the 91XG and I straightened it. It cracked in a couple places, not all the way through. I took a piece of half inch "C" channel and slid it in to the cracked booms. Better than new now.

I also made a new support boom out of my leftover 1 inch square stock with the .125" wall. It's overkill, just the way I like it! Between the "C" Channel inside the long booms and the under boom support, an amazingly strong antenna. I think it will hold up well in the wind.

I did several modifications as I went along. My first mod was to take an extra Center section, remove the front section and add that in between the original Center section and the front section. The signal was Stronger on some channels. Then I added an extra front boom. The results were dramatic. I was very happy with the results. I still have another short rear section that could be added for now I'm happy with what I've got.

I'll let the pictures tell the rest of the story.







Here's the modification to the support boom. The riveted end is for stability.



This end can slide in and out, I found it necessary as the spacing on the directors on the 91 XG is tight:






Boom sag before adding the enhanced boom support:




The last two pics just show the "C" channel that I inserted to reinforce the antenna booms:








]

Last edited by bobsgarage; 1-Jun-2020 at 3:10 PM.
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Old 1-Jun-2020, 4:36 AM   #2
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91XG Spectrum analyzer results

Well, here's the Spectrum analyzer shots. In summary that signal strength went way up. Later this week when my two replacement 91xg scum in I will test them against this modified 91XG antenna





[url=https://ibb.co/k3mVT8q][/url
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Old 1-Jun-2020, 7:48 PM   #3
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Nice work, Bob.

Quote:
Originally Posted by bobsgarage View Post
...I took a piece of half inch "C" channel and slid it in to the cracked booms. Better than new now.

I also made a new support boom out of my leftover 1 inch square stock with the .125" wall. It's overkill, just the way I like it!
Wonder where you obtained the 'C' channel.

Wonder if Home Depot or Lowes has it?

Quote:
Between the "C" Channel inside the long booms and the under boom support, an amazingly strong antenna. I think it will hold up well in the wind.
Yes it probably will hold up fine.

A few years ago, did a similar thing with MCM 30-2370's, but used round tubing inside the boom.
Think it was 3/4 inch square tube to join and reinforce the cradles.

It is located in KY, and has seen some ice and high wind.

.
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Old 1-Jun-2020, 9:37 PM   #4
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Thanks!

We'll call it a 135XG.

Quote:
Originally Posted by tripelo View Post
Nice work, Bob
Thanks!

Quote:
Wonder where you obtained the 'C' channel.

Wonder if Home Depot or Lowes has it?

http://forum.tvfool.com/images/editor/attach.gif

I got it at Home Depot, its for edging or framing plywood. 8 feet was $11.00 with taxes. It's 1/2 inch and slid in quite easily into the 91XG booms except the ones that had gotten bent:




Quote:
Yes it probably will hold up fine.

A few years ago, did a similar thing with MCM 30-2370's, but used round tubing inside the boom.
Think it was 3/4 inch square tube to join and reinforce the cradles.

It is located in KY, and has seen some ice and high wind.
I think my mods will do fine also as far as durability. That 1" box tubing is thick. Sleeving the antenna boom internally seals the deal. BTW, I "bowed" the C channel so it had a bit of spring to it so actually pushes the ends of the boom up a little just to compensate for boom sag. It worked well.



.
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Old 8-Jun-2020, 5:14 PM   #5
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Super "163 XG"

I believe I've achieved my final modification on my Super XG. As you remember I had Taken 2 91XGs and added sections from each other to make a longer "135 XG". It worked well and I probably could have just stopped there.

I couldn't help myself. I saw the extra rear section closest to the dipole left over from the other donor antenna. I looked at spacing and sure enough if you removed every other reflector you could achieve the five and a half inch spacing of the front sections. I actually only had to drill a few holes for the mounting clamps and another because the director spacing wasn't optimal.

After flipping this rear section around and playing with the reflectors I was able to add the rear section to the front. We will call it the "163 XG":



Results were good. I think I have reached the point of diminishing returns. I also know I'm not testing under optimal laboratory type conditions. So I'll do back-to-back scans I tried to keep the test within 10 minutes of each other. Then I'll wait a while and do another back to back scan or even do it the next day. I'll test one and then the other and then the other guy back and forth scales with the same I can say to myself , yeah they are consistent. I always use the same components. Same preamp, same down cable, even the same UVSJ.



Next day:


There was quite a difference between yesterday and today. Although both days were clear, there may have still been some atmospheric conditions contributing to the strength of the scans I did this morning.

Look at the 163XGs signals in less than 24 hours:



Last edited by bobsgarage; 9-Jun-2020 at 2:03 PM.
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Old 10-Jun-2020, 3:19 PM   #6
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Quote:
Originally Posted by bobsgarage View Post
[SIZE="3"]

I looked at spacing and sure enough if you removed every other reflector you could achieve the five and a half inch spacing of the front sections. I actually only had to drill a few holes for the mounting clamps and another because the director spacing wasn't optimal.

After flipping this rear section around and playing with the reflectors I was able to add the rear section to the front. We will call it the "163 XG":
Bob, Interesting modification.

Quote:
...Results were good. I think I have reached the point of diminishing returns.
Yes true.

Still, every small, or not so small, amount of gain adds up.


Quote:
... So I'll do back-to-back scans I tried to keep the test within 10 minutes of each other. Then I'll wait a while and do another back to back scan or even do it the next day. I'll test one and then the other and then the other guy back and forth scales with the same I can say to myself , yeah they are consistent. I always use the same components. Same preamp, same down cable, even the same UVSJ.
Don't know how far you are from the stations?

Have noticed that stations that are LOS (in Dallas) are most stable between hours of about noon and mid-afternoon.

For stations that are considered '2-Edge' and 'tropo', they (in KY) are generally weaker in that same time range.

Quote:
..There was quite a difference between yesterday and today. Although both days were clear, there may have still been some atmospheric conditions contributing to the strength of the scans I did this morning.
Distant signals are usually stronger in mornings.

-----------------------------------

You are discovering the tradeoffs of lengthening Yagis vs stacking. Each has some advantage.

For example:

Ideally, stacking gain could approach 3dB for all the channels the antennas were designed for.

Lengthening a Yagi generally increases gain the most at the highest channels. provides less increase in gain for lower channels.

Contrary to popular opinion, doubling length of a high-gain broadband Yagi does not double gain (3dB) for all channels. For theoretical reasons, doubling length of a normal high-gain Yagi will not double gain for even the high channels.

-----------------------------------

Depending on individual circumstances , there may be good reasons to prefer a long Yagi over a stacked pair, or vice-versa.

You are doing some interesting projects.

.
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Old 10-Jun-2020, 4:40 PM   #7
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Those are some supersized antennas Bob!
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Antennacraft Y10-7-13 VHF, Antennas Direct 91XG UHF
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Old 10-Jun-2020, 8:06 PM   #8
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Long antennas in the wind

Quote:
Originally Posted by Tim View Post
Those are some supersized antennas Bob!
Yes. Mother nature will test them. I will upload a video file to this thread last night, we had some 40 MPH winds, some wobble was going on. The video is just OK.. Night shot:
https://www.dropbox.com/s/xay184dha9...21405.mp4?dl=0
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Old 10-Jun-2020, 8:26 PM   #9
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Thanks Tripelo !

Quote:
Bob, Interesting modification.
Yes true, I had heard that adding a front section was about 1 dB added. I have no idea why others didn't report doing this.

Quote:

Still, every small, or not so small, amount of gain adds up.


Yes. The gain is there and I am happy with the mods...For now




Quote:
Don't know how far you are from the stations?
49 miles south of Milwaukee, in this case. I aim to Milwaukee since it is farther than Chicago. Plus I have a dense maple opposite! Here's my Rabbit Ears report:

https://www.rabbitears.info/searchma...tudy_id=103278

Quote:
Have noticed that stations that are LOS (in Dallas) are most stable between hours of about noon and mid-afternoon.

For stations that are considered '2-Edge' and 'tropo', they (in KY) are generally weaker in that same time range.
I was amazed at the difference in the signals strength overnight. Jeff Kitz told me he was getting some far away stations. From Eagle River he was getting Milwaukee. 242 miles away.

Quote:
Distant signals are usually stronger in mornings.

Such was the case

-----------------------------------

Quote:
You are discovering the tradeoffs of lengthening Yagis vs stacking. Each has some advantage.

For example:

Ideally, stacking gain could approach 3dB for all the channels the antennas were designed for.

Lengthening a Yagi generally increases gain the most at the highest channels. Provides less increase in gain for lower channels.

Contrary to popular opinion, doubling length of a high-gain broadband Yagi does not double gain (3dB) for all channels. For theoretical reasons, doubling length of a normal high-gain Yagi will not double gain for even the high channels.
That would probably make sense in my case although consistently 556 MHz the 163 XG is at least 6 dB better than the 91 XG consistently.
-----------------------------------

Quote:
Depending on individual circumstances , there may be good reasons to prefer a long Yagi over a stacked pair, or vice-versa.

You are doing some interesting projects.
Thanks, It's fun. I am a little disappointed in my stacking attempts though.

I do have 3 brand new 91 XGs ready for testing and one other I can assemble.

I am tempted to play with a quad array but.... I don't know yet, I am going to perfect my dual stack first.

I want to attempt the twin lead lossless balun though.

Believe me, if I could have made the 1/4 wave transformer work, I would have. I will admit, that was really painful.

I think the twin lead endeavor is my best hope for low loss stacking.

I will keep you all informed!




Special ordered the ferrite for the loss less balun two different types, one for VHF and one for UHF:



This is all new to me. Any tips?

Last edited by bobsgarage; 10-Jun-2020 at 10:02 PM.
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Old 11-Jun-2020, 2:00 PM   #10
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Quote:
Originally Posted by bobsgarage View Post
...49 miles south of Milwaukee, in this case. I aim to Milwaukee since it is farther than Chicago. Plus I have a dense maple opposite! Here's my Rabbit Ears report:
Thanks for the RE report.

Looks like you have several strong signal stations to choose from.


Quote:
...That would probably make sense in my case although consistently 556 MHz the 163 XG is at least 6 dB better than the 91 XG consistently.
As you know, there are several other factors that affect received signal strength besides antenna gain.

Quote:
... I am a little disappointed in my stacking attempts though.

I do have 3 brand new 91 XGs ready for testing and one other I can assemble.

I am tempted to play with a quad array but.... I don't know yet, I am going to perfect my dual stack first.

I want to attempt the twin lead lossless balun though.
The ferrite choke balun is actually a 'less-loss' balun. It can be less loss than the common commercial 300:75 balun.

There is no such thing as a lossless balun* (See Note at bottom).


Quote:
...I think the twin lead endeavor is my best hope for low loss stacking.
There are some interesting challenges with twin-lead stacking.

Quote:
...I will keep you all informed!
Thanks.

Quote:
..Special ordered the ferrite for the loss less balun two different types, one for VHF and one for UHF:
Yes, it is better to try to optimize the type of ferrite for the frequency to be rejected.

Quote:
This is all new to me. Any tips?
No really great tips come to mind:

In general, the more ferrite you use, the less loss.

One of the main goals is:

To impede signal loss down the outside of the coax shield by raising impedance as high as practical on the outside of the coax shield.

For example: If one is using only two ferrite cores, it may be best to use one at the junction and one about a quarter wavelength down the coax. If three cores, maybe two at the junction and one a quarter wavelength away. There are other combinations.

Have implemented the separated cores in test installations. Haven't actually tried to measured improvement results, but have run computer simulations that indicate it is better than putting all the cores at the junction.

--------------------

If you go with a quad stack:

- Need to decide of what configuration you want:

1. Diamond Configuration
2. H Configuration
3. All 4 horizontal
4 All 4 Vertical

As usual, some configurations may be more desirable than others.

Be interesting to see how you handle some of difficulties related to use of twin-lead.

Twin lead is a conductor, depending on location, lengths that are longer than about quarter wavelength can resonate and affect antenna patterns.

.




------------------------
Note, baluns and loss:

Some antennas can be built such that the the transformation of 'balanced to unbalanced' occurs in the antenna. Log periodic antennas can be built in this manner. Some specialized folded dipoles can be configured to make the transformation. As such, these transformations are more efficient than most any other method. There are many other types of antennas that provide for an unbalanced (coax) connection and require no external balun.

Last edited by tripelo; 11-Jun-2020 at 2:32 PM. Reason: Add: Note on Baluns
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Old 11-Jun-2020, 10:07 PM   #11
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Great information, thanks Tripleo

Tripleo, thank you for your detailed information. It's exactly what I need. Again as you see I have an eye for detail and I will report each and every change I make. Not just for my benefit or yours but for anybody who may try this. I know it sounds simple but for me it's like working on a new brand of car with new tools and a new scanner.

For now I plan to just try the two antenna stacking side by side. I want to do two 91XGs with the 300 ohm twin lead separate from two 30 - 2476 VHF antennas with a similar. I mean side by side and using the twin lead.


When I am done and I am satisfied I have made good progress , I hope to join them at a combining preamp. I've got a good friend making a low noise VHF UHF combo amp.

So, to stack each array antennas I believe I have ordered the correct ferrite cores. I have 10 of each number, which are Fair-Rite "Ferrite cable cores 61 Z= 280 ohm at 250 megahertz. I believe those are for UHF.

The other ones are the Fair-Rite "Ferrite cable core 43 round cable cores Z = 280 ohms at 100 megahertz". I think those are for VHF.

The plan was to put four on the RG6 coax of the VHF array and four on the RG6 coax of the UHF array with six leftover of each. I'm 100% sure I should have no problem soldering the twin lead to the coax.

Hey, a question came up should I be using dual Shield or quad Shield RG6?

Another friend Sev, is going to do with the similar or the same. The plan was to twist the cable 1 twist per foot or was it two? I'll check my notes. Then put it inside 1/2 " CPVC tubing to keep the rain off of it.

That's the plan I would like to hear any input that you or anyone else has. This is new to me, and I am just learning how to do this.

Oh, and I like your idea of spreading out the ferrite cores. I don't know what the distance would be when you say 1/4 wave, I don't know the calculations. If it's easy for you, I'd like to hear your input on the lengths I need to work with.

I don't mind learning all these calculations but math isn't my strong point. Sure, I know decimals fractions and multiplication / division. But I don't know formulas math and I parted ways at algebra and trigonometry..

Thanks, hope to hear back from you before the weekend when I plan to do this. We're supposed to have some beautiful cooler weather perfect rooftop weather. I'll start with my scans to make sure the two antennas are equal. Which I'm sure will take me several scans to perform an average.

Last edited by bobsgarage; 11-Jun-2020 at 10:16 PM.
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Old 12-Jun-2020, 10:58 AM   #12
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Ferrite, Wavelength, etc.

Quote:
Originally Posted by bobsgarage View Post

For now I plan to just try the two antenna stacking side by side. I want to do two 91XGs with the 300 ohm twin lead separate from two 30 - 2476 VHF antennas with a similar. I mean side by side and using the twin lead.
Not clear how your choke balun is configured?

The choke balun does not make impedance transformation as regular 300:75 Ohm baluns would.

For example; two 300-Ohm antennas paralleled would provide about 150 Ohms impedance at the junction. Then a 150:75 Ohm transformation is needed. Then the ferrite choke balun could be applied.

Another example; The twin lead from four 300 Ohm identical antennas could be be paralleled to provide about 75 Ohms impedance, then the ferrite choke balun applied.

Quote:
...So, to stack each array antennas I believe I have ordered the correct ferrite cores. I have 10 of each number, which are Fair-Rite "Ferrite cable cores 61 Z= 280 ohm at 250 megahertz. I believe those are for UHF.

The other ones are the Fair-Rite "Ferrite cable core 43 round cable cores Z = 280 ohms at 100 megahertz". I think those are for VHF.
As you know, UHF is 470-608 MHz. Upper VHF is 174-216 MHz.

Haven't looked at your particular ferrites. It is best to select for maximum impedance at the frequencies of interest.

For UHF, In future you might want take a look at Laird (PN 28B0616-000), it is solid and fits over regular RG6 (not quad), provides more than 500 Ohms impedance at 500 MHz. It has fairly high impedance at upper VHF as well. Maybe the Fair-Rites you have are similar, one could compare graphs.





The Laird ferrites are available at DigiKey. Think Mouser has them also. Laird also has some other sizes that are convenient, such as to fit over Connectors, etc.

Quote:
Hey, a question came up should I be using dual Shield or quad Shield RG6?
Unless you have some very strong local interfering next to, or very close to the coax, the quad shield version is not helpful. Quad shield does not have any advantage with respect to attenuation, it is stiffer to work with, and usually requires a different connector than regular coax.

Quote:
Another friend Sev, is going to do with the similar or the same. The plan was to twist the cable 1 twist per foot or was it two? I'll check my notes.
The main reason for the twist is to help reject pickup of strong signals by the twinlead.

Optimal twist depends on frequencies to be rejected. It is usually not critical.


Quote:
...spreading out the ferrite cores. I don't know what the distance would be when you say 1/4 wave, I don't know the calculations...
TV & radio signals travel thru space as electromagnetic waves, like light.

The speed of light is about 300 million meters per second.

The frequency is in Hertz, that's cycles per second (cycles being wavelengths).

The length of a wavelength is the distance traveled in one second, divided by the number of cycles in one second.

For example:

At 535 MHz, that’s 535 million cycles per second, that’s also 535 million wavelengths per second.

Divide 300 million meters per second by 535 million cycles per second

The ‘per seconds’ apply to both numerator and denominator so they cancel.
The ‘million’ apply to both numerator and denominator, so they cancel.

That leaves meters as the units.

300/535=0.56 meters

There are 39.37 inches per meter.

0.56 meters ~= 22 inches

One fourth of 22 inches ~ = 5.5 inches

There are two numbers to remember; Speed of light and number of inches in a meter.

It is not necessary to apply a velocity factor.

Since signal of interest is traveling on the coax shield, then conductor is outside the coaxial cable and the signal travels at nearly the speed of light.

If inside a coaxial cable, the velocity of propagation slows. The slowed signal can travel at speeds in the range of about 66% to maybe 85% for common coax. For twinlead the slowing effect is less pronounced, maybe in the range of 80-95%.

Because the ferrites are an inch or more in length, that occupies a significant portion of a wavelength at UHF, so the precise location of a quarter wavelength is not critical.

For upper VHF, multiply the above number by 3 might be close enough, or recalculate to get about; 15 inches for quarter wavelength.

Or,

Plug frequencies in to this (or other) online wavelength calculator:

http://www.procato.com/calculator-wavelength-frequency/

.
Attached Images
File Type: gif 28B0616-000-IC.gif (14.6 KB, 259 views)
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Old 12-Jun-2020, 1:43 PM   #13
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Quote:
Originally Posted by tripelo View Post
Not clear how your choke balun is configured?

The choke balun does not make impedance transformation as regular 300:75 Ohm baluns would.

For example; two 300-Ohm antennas paralleled would provide about 150 Ohms impedance at the junction. Then a 150:75 Ohm transformation is needed. Then the ferrite choke balun could be applied.

Another example; The twin lead from four 300 Ohm identical antennas could be be paralleled to provide about 75 Ohms impedance, then the ferrite choke balun applied.



As you know, UHF is 470-608 MHz. Upper VHF is 174-216 MHz.

Haven't looked at your particular ferrites. It is best to select for maximum impedance at the frequencies of interest.

For UHF, In future you might want take a look at Laird (PN 28B0616-000), it is solid and fits over regular RG6 (not quad), provides more than 500 Ohms impedance at 500 MHz. It has fairly high impedance at upper VHF as well. Maybe the Fair-Rites you have are similar, one could compare graphs.





The Laird ferrites are available at DigiKey. Think Mouser has them also. Laird also has some other sizes that are convenient, such as to fit over Connectors, etc.



Unless you have some very strong local interfering next to, or very close to the coax, the quad shield version is not helpful. Quad shield does not have any advantage with respect to attenuation, it is stiffer to work with, and usually requires a different connector than regular coax.



The main reason for the twist is to help reject pickup of strong signals by the twinlead.

Optimal twist depends on frequencies to be rejected. It is usually not critical.




TV & radio signals travel thru space as electromagnetic waves, like light.

The speed of light is about 300 million meters per second.

The frequency is in Hertz, that's cycles per second (cycles being wavelengths).

The length of a wavelength is the distance traveled in one second, divided by the number of cycles in one second.

For example:

At 535 MHz, that’s 535 million cycles per second, that’s also 535 million wavelengths per second.

Divide 300 million meters per second by 535 million cycles per second

The ‘per seconds’ apply to both numerator and denominator so they cancel.
The ‘million’ apply to both numerator and denominator, so they cancel.

That leaves meters as the units.

300/535=0.56 meters

There are 39.37 inches per meter.

0.56 meters ~= 22 inches

One fourth of 22 inches ~ = 5.5 inches

There are two numbers to remember; Speed of light and number of inches in a meter.

It is not necessary to apply a velocity factor.

Since signal of interest is traveling on the coax shield, then conductor is outside the coaxial cable and the signal travels at nearly the speed of light.

If inside a coaxial cable, the velocity of propagation slows. The slowed signal can travel at speeds in the range of about 66% to maybe 85% for common coax. For twinlead the slowing effect is less pronounced, maybe in the range of 80-95%.

Because the ferrites are an inch or more in length, that occupies a significant portion of a wavelength at UHF, so the precise location of a quarter wavelength is not critical.

For upper VHF, multiply the above number by 3 might be close enough, or recalculate to get about; 15 inches for quarter wavelength.

Or,

Plug frequencies in to this (or other) online wavelength calculator:

http://www.procato.com/calculator-wavelength-frequency/

.
Good morning Tripelo.

The harness he is attempting emulate is based on the one Calveras built some time back.
I am working on assembling a couple of quad arrays in diamond form using the same method.

The beads selected are 61 for UHF and 43 for VHF if I recall correctly off the top of my head. From the Mouser catalog.
These were the sizes suggested by Calveras.

His harness was 4 phase matched lengths of twin soldered to a length of Belden RG6 with 4 ferrite beads back to back starting at the soldered juncture.

For my purposes I will be using 4 DB4E panels for 1 quad array and 4 yagi's from Newark that are the HDB91x sold on solid signal

I recently came into a NOS Channel Master Spartan 3 0064DSB with the 300 Ohm input.
I am hoping this will allow me to forgo the need for the beads and coax as the output on the Spartan is 75 Ohm.
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Old 12-Jun-2020, 2:40 PM   #14
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Good morning Sev.

Quote:
Originally Posted by Sev View Post

...I recently came into a NOS Channel Master Spartan 3 0064DSB with the 300 Ohm input.
I am hoping this will allow me to forgo the need for the beads and coax as the output on the Spartan is 75 Ohm.
The CM 0064DSB has a regular ferrite 300:75 Ohm balun at its input.

The core and windings are seen in the lower right of this image (near input terminals).






Could be CM used a slightly better core than used in most commercial baluns. However, the balun has loss.

Removed the core and windings, replaced with half-wave loop saw signal about 1-2 dB strength improvement.

.


No offense Sev, the post was quick and for a while was thinking about Bob's earlier posts.
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File Type: jpg CM-0064 Circuit Components.jpg (44.7 KB, 231 views)

Last edited by tripelo; 12-Jun-2020 at 3:07 PM. Reason: Good morning Sev, and Bob
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Old 12-Jun-2020, 4:33 PM   #15
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Quote:
Originally Posted by tripelo View Post
Good morning Sev.



The CM 0064DSB has a regular ferrite 300:75 Ohm balun at its input.

The core and windings are seen in the lower right of this image (near input terminals).






Could be CM used a slightly better core than used in most commercial baluns. However, the balun has loss.

Removed the core and windings, replaced with half-wave loop saw signal about 1-2 dB strength improvement.

.


No offense Sev, the post was quick and for a while was thinking about Bob's earlier posts.
No offense at all. More information is always better.

By the by. Perhaps you would know. As anybody ever added the missing components on that board to switch it to a 0264?

Would have have a picture of the 0264 board? I have yet to come across one.
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Old 12-Jun-2020, 4:55 PM   #16
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CM-0264 Preamp

Quote:
Originally Posted by Sev View Post
...

As anybody ever added the missing components on that board to switch it to a 0264?
Seems feasible.

Main item is additional balun.

PCB traces are already there. Need to cut an etch connecting UHF & VHF, etc. Probably have to solder in a small jumper.

Maybe touch up tuning on input coils.

Quote:
...Would have have a picture of the 0264 board? I have yet to come across one.
Yes, below, CM-0264 Circuit components:





.
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Last edited by tripelo; 12-Jun-2020 at 5:13 PM.
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Old 12-Jun-2020, 6:04 PM   #17
bobsgarage
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Ferrite cores

Tripelo, thanks.

I am happy that you have given me this info

My head is spinning but I will recover.




Here are the Ferrite cores ordered from Mouser:

UHF:
https://www.mouser.com/ProductDetail...smXY4HC8Iew%3D

VHF:
https://www.mouser.com/ProductDetail...tkRaXC%2F3o%3D

Man, they are so very careful about the packaging. I guess they are very brittle.

Are you thinking I only need two Ferrite cores or, the more the merrier?

At least one at the junction and at least one at the 1/4 wave length, 5.5" down?


I have 10 of each of the above.

I need to stop and get some more shrink wrap

Last edited by bobsgarage; 12-Jun-2020 at 6:49 PM.
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Old 12-Jun-2020, 8:29 PM   #18
tripelo
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Quote:
Originally Posted by bobsgarage View Post
Tripelo, thanks.

Here are the Ferrite cores ordered from Mouser:
The graphs of ferrite impedance look OK for your application.

Quote:
Man, they are so very careful about the packaging. I guess they are very brittle.
They are a form of ceramic, if they bang against each other they crack and separate.

Quote:
..Are you thinking I only need two Ferrite cores or, the more the merrier?
Merry is a good state.

Quote:
At least one at the junction and at least one at the 1/4 wave length, 5.5" down?
Looks like your UHF Fair-Rite's have somewhat more than about 400 Ohms impedance at 500 MHz. If you use four of them, that makes at least 1600 Ohms (from ferrite) to resist signal passage down the coax shield. Splitting locations, 2 at junction and two at 1/4 wave would likely increase the effective impedance to well above 1600 Ohms.

Four of them together at 1.126 inches length each would cover about 4.5 inches. Splitting for quarter wave leaves only a small space between.

The signal can divide some portion to pass through the interior of the coax at 75 Ohms impedance, some to pass through 1600+ Ohms (plus the inherent impedance down a coax shield outer conductor).

The point is, the higher the impedance, the less signal passes on the outer shield. Signal on the outside of the shield is for practical purposes 'lost'.

There are practical limits, cost, weight, difficulty handling and securing a ferrite laden cable.

In some cases, two or three is about all a connection at a junction can handle.

Diminishing returns, revisited.


.
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Old 12-Jun-2020, 9:19 PM   #19
bobsgarage
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Diminishing Returns and

Quote:
Originally Posted by tripelo View Post
The graphs of ferrite impedance look OK for your application.

They are a form of ceramic, if they bang against each other they crack and separate.

Merry is a good state.
I will be merry when I get the results I want

Quote:
Looks like your UHF Fair-Rite's have somewhat more than about 400 Ohms impedance at 500 MHz. If you use four of them, that makes at least 1600 Ohms (from ferrite) to resist signal passage down the coax shield. Splitting locations, 2 at junction and two at 1/4 wave would likely increase the effective impedance to well above 1600 Ohms.

Four of them together at 1.126 inches length each would cover about 4.5 inches. Splitting for quarter wave leaves only a small space between.
So at least for UHF, I will have four ferrite cores on the coax? OK so far so good. Will I be limited to the length on the coax?

And what about the VHF? That should be a longer 1/4 wave right?

How should I configure the ferrite on that coax?

Quote:
The signal can divide some portion to pass through the interior of the coax at 75 Ohms impedance, some to pass through 1600+ Ohms (plus the inherent impedance down a coax shield outer conductor).

The point is, the higher the impedance, the less signal passes on the outer shield. Signal on the outside of the shield is for practical purposes 'lost'.

There are practical limits, cost, weight, difficulty handling and securing a ferrite laden cable.

In some cases, two or three is about all a connection at a junction can handle.

Diminishing returns, revisited.
Weight seems like the most important concern. No reason to put strain on my Kitz KT-200 F fitting. Or, the strain on the F connectors. Or the extra weight pulling on the Dipole, although I will be encasing the twin lead in CPVC tube to keep rain off and that should relieve the strain at the dipole end of the twin lead.


In my case, the cost is low so that's not an issue.
So, 2 or 3 is enough? 4 is OK, but wont do much? As in "diminishing returns"?...


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Old 12-Jun-2020, 10:06 PM   #20
tripelo
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Quote:
Originally Posted by bobsgarage View Post
I will be merry when I get the results I want

So at least for UHF, I will have four ferrite cores on the coax? OK so far so good. Will I be limited to the length on the coax?
There is no absolute limit relative to length. Probably way down the coax would have little effect. You will be limited by the length of coax to your preamplifier.

Two ferrites, next to the junction and two ferrites centered over the 5.5 inch mark should be fine. That would mean the edge of one ferrite on the antenna side of a 5.5 inch mark and another one next to it down the coax.

Quote:
And what about the VHF? That should be a longer 1/4 wave right?
Yes, assuming your cable to preamp is 15 inches in length, or longer.

Quote:
...How should I configure the ferrite on that coax?
If you use four ferrites. Place two near the junction close together. Place the other two centered around 15 inches down the cable.

If you use three, then place two at the junction and one at 15 inches.

Quote:
So, 2 or 3 is enough? 4 is OK, but wont do much? As in "diminishing returns"?...
With calculations or computer simulations one could obtain an estimate of the available improvement, or indirectly measure it on a test bench.

After two or three ferrite cores on a cable, likely the signal strength improvement available with more ferrite is in the low tenths of a dB range.


.

Last edited by tripelo; 12-Jun-2020 at 10:10 PM. Reason: clarify
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