TV Fool  

Go Back   TV Fool > Over The Air Services > Special Topics > Antennas

Notices

Reply
 
Thread Tools
Old 12-Jun-2020, 10:06 PM   #21
Sev
Member
 
Join Date: Jan 2016
Posts: 81
Quote:
Originally Posted by tripelo View Post
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.



Yes, below, CM-0264 Circuit components:





.
Here is mine.
Looks like the L7A is the bridge. Or no?

Sev is offline   Reply With Quote
Old 12-Jun-2020, 10:18 PM   #22
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 157
Quote:
Originally Posted by Sev View Post
Here is mine.
Looks like the L7A is the bridge. Or no?
Yes, looks like it is.

It appears L7 is meant to move to L7a location for the 0264 version.

Your preamp is probably newer.

.
tripelo is offline   Reply With Quote
Old 13-Jun-2020, 12:42 AM   #23
bobsgarage
Senior Member
 
bobsgarage's Avatar
 
Join Date: Mar 2016
Location: Beach Park IL
Posts: 296
The "Less Loss" harness begins...

Quote:
Originally Posted by tripelo View Post
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.



Yes, assuming your cable to preamp is 15 inches in length, or longer.



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.



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.


.
Tripelo,

Wow ! OK, I have to say I am pretty fired up!

My ADD (Antenna Distraction Disorder) is kicking in.

You have made it very clear for me- a novice. This will help others too I'm sure, if they take the time to read it.

Oh, the twists in the twin Lead, Sev was telling me, it's up to me how many twists, just make sure they are the same amount per side. That may be the trickiest part.
bobsgarage is offline   Reply With Quote
Old 13-Jun-2020, 11:39 AM   #24
bobsgarage
Senior Member
 
bobsgarage's Avatar
 
Join Date: Mar 2016
Location: Beach Park IL
Posts: 296
Quote:
Originally Posted by tripelo View Post
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.



Yes, assuming your cable to preamp is 15 inches in length, or longer.



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.



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.


.
Tripleo thank you. It's all very clear to me now. I will take it step-by-step pictures, and I will report back. I also want to thank my other friends sheer on TV Fool and the other forums. Sev, Rabbit, Tim and Calaveras.

Seems like I always pick something complicated to do and it's hard to get input that way. So, I really appreciate it.

Last edited by bobsgarage; 13-Jun-2020 at 12:26 PM.
bobsgarage is offline   Reply With Quote
Old 13-Jun-2020, 11:46 AM   #25
Sev
Member
 
Join Date: Jan 2016
Posts: 81
Quote:
Originally Posted by tripelo View Post
Yes, looks like it is.

It appears L7 is meant to move to L7a location for the 0264 version.

Your preamp is probably newer.

.
Could be. There are some noticeable differences between the two.

Bare copper vs coated copper.
It also would appear they tuned the G1 next to the trap adjustment.
It has a silicon strip across it to hold the coils in place.

If somebody had a CNC machine to mill the traces. That board would be pretty easy to replicate after inputting the necessary data.

Then it would be a matter determining the ferrite core. Or install half wave loops and sourcing the rest of the components. It would be a nice challenge for a hobbyist.

The units have 3.0 and 2.2 dB in noise. I wonder if it would be possible to determine and optimize the components creating the noise.

Jeff Kitz has shown that it is possible to create low noise preamp. Then there is Winegards LNA-100. Also very low noise.
I would imagine a portion of the noise is being generated by the PSU.
The 3 capacitors @ C7, C13 and C28 would be suspect as well.
Sev is offline   Reply With Quote
Old 14-Jun-2020, 6:35 PM   #26
bobsgarage
Senior Member
 
bobsgarage's Avatar
 
Join Date: Mar 2016
Location: Beach Park IL
Posts: 296
Quote:
Originally Posted by bobsgarage View Post
Tripleo thank you. It's all very clear to me now. I will take it step-by-step pictures, and I will report back. I also want to thank my other friends sheer on TV Fool and the other forums. Sev, Rabbit, Tim and Calaveras.

Seems like I always pick something complicated to do and it's hard to get input that way. So, I really appreciate it.
Yeah, I spoke too soon. I guess I wasn't clear on how many antennas I was trying to stack. I'm only looking to stack two at this time. I had some rude awakening when I was PMd.

"OK, I understand about the ferrite to form a choke balun around the coax, but there are two questions I have in mind: 1. If you connect two 300 twinleads together in parallel, that would give you 150 ohms. Wouldn't that be a 2:1 missmatch with the 75 ohm coax? 2. When you connect the two twinleads in parallel, they must be in phase. It's like connecting two batteries in parallel: + to + and - to -. How will you know which way is correct?It will work, but there will be some loss because of the mismatch which causes an SWR of 2:1. It is difficult for me to predict how much additional gain two antennas connected that way will give you compared to just one antenna.
There are two problems to be concerned about when you connect together in parallel the two 300 ohm lines from two antennas.
The first problem is the mismatch loss because two 300 ohm lines in parallel will give you 150 ohms, which you will then connect to a 75 ohm coax line.
You aren't going to like to hear this, but you would need a 1/4 wave matching section of 106 ohms between the 150 ohm point and the 75 ohm coax for a perfect match.
The second problem is getting the two 300 ohms lines in phase (or polarity if you prefer) when they are connected together in parallel. You have a 50/50 chance of getting it right. If they are connected out of phase the main lobe will split in two like this"


https://imageevent.com/holl_ands/sta...f5v5sec1.goose
Ooops... looks like I have another problem to solve? Now that we're in the 11th hour I'm so glad that I did not execute my plan fully. I'm kind of glad it was chilly Friday after work and I had to do some deck work around the house. That kept me off the roof! Anyhow what should I do? What do I do to achieve the correct match?
bobsgarage is offline   Reply With Quote
Old 14-Jun-2020, 7:21 PM   #27
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 157
Quote:
Originally Posted by Sev View Post
The units have 3.0 and 2.2 dB in noise.
Yes, plus.

Channel Master and some other manufacturers have not provided accurate noise figure numbers.

Numbers often provided are quotes from transistor manufacturers specs.

There is more to noise figure than transistor specs, for example:

- The excellent input filtering of the 0264/0064 and other older CM preamps, contribute to the noise figure.
- The loss in the ferrite balun adds to the noise figure.

Quote:
I wonder if it would be possible to determine and optimize the components creating the noise.
The VHF amplifier is a BFG198.

The first UHF transistor is a BFG67

(1st transistor is usually main active source of noise)

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

The 0064 & 0264 are good and reliable preamplifiers.

The design, except for input baluns, is virtually the same as for the old CM7778.

The old CM7778 was cloned to the RCA_TVPRAMP1R.

http://forum.tvfool.com/showpost.php...76&postcount=2


.
tripelo is offline   Reply With Quote
Old 14-Jun-2020, 10:43 PM   #28
OTAFAN
Senior Member
 
Join Date: Apr 2015
Posts: 216
Quote:
Quote:
Originally Posted by Sev View Post
Jeff Kitz has shown that it is possible to create low noise preamp. Then there is Winegards LNA-100. Also very low noise.
I would imagine a portion of the noise is being generated by the PSU.
The 3 capacitors @ C7, C13 and C28 would be suspect as well.
An old tip that was passed on to me decades ago by an electronics audiophile, was to replace all the CAPS in the audio path of an AM/Shortwave (or any other) radio with non-polarized CAPS to enhance the clarity and lower the noise floor of the receiver. I can say it always improved my audio with less aural fatigue. I think some high end receivers in the past have also used non-polarized CAPS in their audio paths.

I haven't tried it in situations like this thread is addressing, but I suspect it might be worth experimenting to lower noise.

One advantage of non-polarized CAPS is that is doesn't matter which way you solder them in.

Also, they're not too expensive as compared to regular CAPS and can still be bought at places like Mouser Electronics:

https://www.mouser.com/Search/Refine...zed+capacitors

You Techs here are above my pay grade, but I hope this might be of some help.

Last edited by OTAFAN; 15-Jun-2020 at 7:42 AM. Reason: additional
OTAFAN is offline   Reply With Quote
Old 15-Jun-2020, 12:25 PM   #29
Sev
Member
 
Join Date: Jan 2016
Posts: 81
Quote:
Originally Posted by tripelo View Post
Yes, plus.

Channel Master and some other manufacturers have not provided accurate noise figure numbers.

Numbers often provided are quotes from transistor manufacturers specs.

There is more to noise figure than transistor specs, for example:

- The excellent input filtering of the 0264/0064 and other older CM preamps, contribute to the noise figure.
- The loss in the ferrite balun adds to the noise figure.



The VHF amplifier is a BFG198.

The first UHF transistor is a BFG67

(1st transistor is usually main active source of noise)

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

The 0064 & 0264 are good and reliable preamplifiers.

The design, except for input baluns, is virtually the same as for the old CM7778.

The old CM7778 was cloned to the RCA_TVPRAMP1R.

http://forum.tvfool.com/showpost.php...76&postcount=2


.
I assume both of the BFG components are on the back side of the board?
This image popped up on google.
A thread with you and Pete Higgins.
https://forum.tvfool.com/showthread.php?t=13530

Do you know of any alternative componets that would lower the noise figure. Or based on the design it is baked in?



One of the members a long time ago mentioned to me that the RCA was a clone of the 7778. The first preamp I picked up actually.

So based on Calveras's testing. The CM's and RCA's had a noise figure very similar to the Blonder Tongue Galaxy III's. Assuming that B.T. was being truthful about their noise levels.
I have come across a few B.T. 300ohm dual inputs with the 75ohm output but passed no them because there did not appear to be much love for them due to their advertised noise figures.

Any idea if those figures are actually accurate or not?

To clarify a statement back on post #14 you stated you removed the ferrite bead and replaced it with a half-wave loop on a Spartan?

OTAFAN posted an interesting idea concerning replacing the polarized caps with non polarized.

Thoughts?

Modding the board should not be very difficult. Though I would like to find another Spartan to test on.

Last edited by Sev; 15-Jun-2020 at 12:59 PM.
Sev is offline   Reply With Quote
Old 15-Jun-2020, 12:30 PM   #30
Sev
Member
 
Join Date: Jan 2016
Posts: 81
Quote:
Originally Posted by OTAFAN View Post
An old tip that was passed on to me decades ago by an electronics audiophile, was to replace all the CAPS in the audio path of an AM/Shortwave (or any other) radio with non-polarized CAPS to enhance the clarity and lower the noise floor of the receiver. I can say it always improved my audio with less aural fatigue. I think some high end receivers in the past have also used non-polarized CAPS in their audio paths.

I haven't tried it in situations like this thread is addressing, but I suspect it might be worth experimenting to lower noise.

One advantage of non-polarized CAPS is that is doesn't matter which way you solder them in.

Also, they're not too expensive as compared to regular CAPS and can still be bought at places like Mouser Electronics:

https://www.mouser.com/Search/Refine...zed+capacitors

You Techs here are above my pay grade, but I hope this might be of some help.
LOL. I dont think I have ever been accused of being a tech.
Interesting idea. Need more data on it to see what the possibility is.

I knew guys years ago that were audiophiles. They were just as bad if not worse than the car guys trying to squeak out every very last bit of HP out of their engines.
Sev is offline   Reply With Quote
Old 15-Jun-2020, 3:34 PM   #31
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 157
Quote:
Originally Posted by Sev View Post
I assume both of the BFG components are on the back side of the board?
Yes

Quote:
Do you know of any alternative componets that would lower the noise figure. Or based on the design it is baked in?
The 1st UHF transistor (BFG67) could be replaced with a BFG520.

It might lower noise figure a couple of tenths of a dB.

Prrobably need to retune.

The transistor substitution works fine, did that when replacing balun.

Quote:
…I have come across a few B.T. 300ohm dual inputs with the 75ohm output but passed no them because there did not appear to be much love for them due to their advertised noise figures.

Any idea if those figures are actually accurate or not?
Based on the circuit design, the components, and comparative testing:

The older 0064/0264 have about the same noise figure as the BT Galaxy series.

Quote:
To clarify a statement back on post #14 you stated you removed the ferrite bead and replaced it with a half-wave loop on a Spartan?
Image of CM0064 with half-wave balun below:




The coax is RG-180 (95 Ohm). The impedance is not ideal, but in some ways better than 75 Ohm coax.
It could be better if the impedance for the loop was 150 Ohms.
But, such cable would have to be custom made and it would probably be too large to fit in the case.


Quote:
OTAFAN posted an interesting idea concerning replacing the polarized caps with non-polarized.

Thoughts?
Doesn’t apply to RF amplifiers.

Some audio circuit paths require passage of bipolar signals (plus & minus).

Audio signals are very low frequency relative to RF (long wavelength, calculate wavelength for say 20 Hertz),
therefore large capacitors are needed.

All bipolar signals in an RF amplifier pass through small ceramic capacitors, they are essentially nonpolar.

The polarized capacitors in RF amplifiers are for filtering the DC (not bipolar) supply voltages.

The noise that is bothersome in preamplifiers is usually in the same frequency range as the desired signals (MHz).

.
Attached Images
File Type: jpg CM0064 w Half Wave Balun.jpg (46.8 KB, 141 views)
tripelo is offline   Reply With Quote
Old 15-Jun-2020, 4:52 PM   #32
Sev
Member
 
Join Date: Jan 2016
Posts: 81
Quote:
Originally Posted by tripelo View Post
Yes



The 1st UHF transistor (BFG67) could be replaced with a BFG520.

It might lower noise figure a couple of tenths of a dB.

Prrobably need to retune.

The transistor substitution works fine, did that when replacing balun.



Based on the circuit design, the components, and comparative testing:

The older 0064/0264 have about the same noise figure as the BT Galaxy series.



Image of CM0064 with half-wave balun below:




The coax is RG-180 (95 Ohm). The impedance is not ideal, but in some ways better than 75 Ohm coax.
It could be better if the impedance for the loop was 150 Ohms.
But, such cable would have to be custom made and it would probably be too large to fit in the case.






The noise that is bothersome in preamplifiers is usually in the same frequency range as the desired signals (MHz).

.
Hmmmmm.
I assume that 150ohm Belden Twinax would not be appropriate for use for the half loop?
Sev is offline   Reply With Quote
Old 15-Jun-2020, 5:46 PM   #33
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 157
Quote:
Originally Posted by Sev View Post
Hmmmmm.
I assume that 150ohm Belden Twinax would not be appropriate for use for the half loop?
The Twinax is shielded twinlead. Like your 300 Ohm twinlead with a shield. It could be useful in some situations.

It probably could be made to work for a half wave loop balun. but would require some unconventional considerations.

In general, coax is better for the half wave loop.

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

Referring to the 0064 with half-wave balun in previous image.

The input & output of VHF portion was disconnected and powered down.

There is some noise figure advantage in not having a VHF circuit connected to a UHF circuit.

Anyhow, that balun would not work well for VHF.

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

FYI, will not be logging in frequently for a while.

.
tripelo is offline   Reply With Quote
Old 15-Jun-2020, 7:21 PM   #34
Sev
Member
 
Join Date: Jan 2016
Posts: 81
Quote:
Originally Posted by tripelo View Post
The Twinax is shielded twinlead. Like your 300 Ohm twinlead with a shield. It could be useful in some situations.

It probably could be made to work for a half wave loop balun. but would require some unconventional considerations.

In general, coax is better for the half wave loop.

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

Referring to the 0064 with half-wave balun in previous image.

The input & output of VHF portion was disconnected and powered down.

There is some noise figure advantage in not having a VHF circuit connected to a UHF circuit.

Anyhow, that balun would not work well for VHF.

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

FYI, will not be logging in frequently for a while.

.
Thanks for the info.

Hope all is well on your end.
Sev is offline   Reply With Quote
Reply

Bookmarks

Go Back   TV Fool > Over The Air Services > Special Topics > Antennas


Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off




All times are GMT. The time now is 11:01 PM.


Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Copyright © TV Fool, LLC