1/4 Wave Matching Sections and Combining Antennas

[bobsgarage]

Well, so far it's not a success story. Far from it I'd say. I have been reading all along the best way to combine antennas is simply not using a combiner /splitter. From what I understand, you're lucky to get anything out of combining the two antennas if you use a splitter to combine them. Well, to me that's just stupid.


So as far as I understand, the most common now it's things are in order to combine 2 antennas for gain, you must have several important features present.

#1. Both antennas must be exactly the same, from the same manufacturer, part number. Essentially the same production.

#2. Most coax cables must be the exact same length, the same kind for example : RG6 dual shield Etc... and should be the same manufacturer and part number. Might as well take it off the same spool.

#3. Both antennas must be mounted at the same height but can be stacked over each other with the proper distance or side by side, where the separation isn't a big deal.

#4. Both antennas need to be aimed at exactly the same target in order to make the gain.

#5. There's no point combining antennas for gain if you are going to have a lot of loss in the system. Combiners/splitters eat signal. So, this is where the quarter wave 50 ohm "matching" (stub) section comes in handy.

So here's my saga. Get a beer, some popcorn relax and read on.

So I did my research, talked to a few people on some other forums and found out that either RG 8 or RG58 can be used to create these matching sections. As longs as it's 50 ohm. A friend of mine who I met on this forum just happened to have a roll of foam filled RG8. StereoCraig has donated several lengths that I thought I would never need. Honestly for my new arrays, one VHF pair and one UHF pair I figured that would be enough. However it didn't turn out that way.

I had first read that quarter wave matching section for UHF made from RG 8 should be 10 inches. Including connectors. Okay. No problem, I butchered a combiner, got the cover off tore all the guts out and ran my two equal antenna leads into the sides stripped the RG 8 and somehow got it to slip into a RG 11 coax connector. (I drilled it). I then used some solder to join the three together inside the combiner. It was my first time so it took me sometime, but it seemingly turned out fine.

[bobsgarage]

So, I climb the 22 foot ladder. I install my new piece I'm so excited! I carefully measure the spacing and aim my 91XG's. There are no leaves on the trees yet. I check all my connections, climb down the ladder and run a scan. What? How can this be. I run the scan again, I climb back up the ladder I take everything back apart I retighten. Nope.

I find out later that I made my matching section too short. 10 in is wrong for foam filled RG 8. That's for polyethylene filled RG 8. By doing the calculations, my trusted friend Rabbit figures out for me that it should be 12 in because the velocity Factor is different. Foam filled is slightly faster then the other. No problem that's all I need to do I'll make another section. This time I decided not to play around with combiner boxes I figured out a way to make an RG8 cable work, look and install just like a piece of RG 11.

I make my 12" RG 8 matching section. Back up the ladder, I need the exercise anyhow. I attach it run down in my basement and run my scans. Another disaster. I recheck all my connections, I'm back up the ladder several times trying to find out what the problem is. This just isn't working.

Okay, brain fart. Guess what I forgot? I forgot to tell the guys it was UHF that I would be combining when I got the measurements? I assumed the matching section would be the same.. Originally, the measurements I got were for a VHF matching section. However, combining my VHFs didn't go so smoothly either. Another story. Maybe that'll be another post for another day. Or I'll incorporated into this thread.

Since I didn't know the matching sections would be different between UHF and VHF, my mistake and because I had done the exact same thing with my VHF a few weeks earlier with no success and I had several different lengths of matching sections between 9 and 1/2 in + 13 and 1/2 in. None of it worked.



So, I find out that for UHF, its 3 and 1/2 inch RG 8 matching section if it's polyethylene insulated but it's 4.6" if it's foam filled. I'm thinking to myself, no wonder I had such poor results! My matching sections were three times longer than they needed to be!
I make a new matching section stub.

I climb the ladder, I'm pumped! I take the 12" matching section off of my Kitz KT200 amp. By the way, I don't know if you've ever taken a coax out of a Kitz Tech amp but it's not easy with your fingertips. Especially RG 11. It's an awesome designed Pre-amp but it takes some time to master reaching inside of it and starting the RG11 cable connectors especially when these are the Belden Aqua Tights. I've literally done this 50 times in the last 2 weeks with different testing I'm still not good at it. And you think I would have bent the 7/16" wrench by now.



Okay, where was I? Oh yeah, I put my freshly made 4.75" matching section in, checked all my connections twice, ran down in my basement and ran the scans again. Disaster strikes again.

After several private messages to different friends, it was suggested that maybe, just maybe I'm not getting the shielding into the connector correctly, yes I have been trimming shielding back because the connector goes deep under the skin of the RG 8 anyhow where I'm sure the connector makes contact with the shielding. No problem so here it is Memorial Day. Since it started out raining this morning I decided I would make one more attempt at making a 4.66" matching section.

I mentioned this before but even before I made my first matching section I was told to include the length of the connectors. However it was never fully understood that I was adapting an RG-11 connector to the RG-8. You know, inside the RG-11 connectors there's some tip that forms the actual contact whatever you're screwing the cable onto. So I figured I would outsmart myself and make this one longer. Because really, isn't it the function of RG8 length that matters?

So this time, I decided to make the cable itself 4 and 1/2 inches long and then solder on the contact tips from the RG-11 connectors. Now, the overall length is 6.5" .





However, this didn't work either. I'm at a loss.. Could it be the "T" connectors?

Here you can see that the two A.D. 91 XG's are not equal. I did call A.D. and they sent out a new balun, more on that later.



It's not the cables and although antennas aren't really balanced, noted during previous testing a simple Holland GHS Pro splitter outperforms any type of matching wave section I have conjured up.

[bobsgarage]

I'm just wondering if my screenshots are showing in the post above? When I save the post they were all there

[tripelo]

Originally Posted by bobsgarage

I'm just wondering if my screenshots are showing in the post above? ...they were all there

They?

See one image. It is open combiner case.

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

There could be other difficulties, but a noticable one is:

A difficulty with combining UHF with Q-wave matching is the union of the three cables.
All parameters are more critical at UHF, than VHF.

Inside the open case, the impedance is no longer 50 Ohms or 75 Ohms.
Therefore the correct impedance match of 37.5 Ohms (parallel combo)
to 75 Ohms via the Q-line is not occurring.

Since at VHF the dimensions (wavelength related) are about three times larger,
then the mismatch caused by impedance bump at the joint has less effect (although it is still there).

Best to join all cable as closely as possible (including shields).
Maybe could do that by drilling the case and running shields all in.
That means very short distances of cable that are not fully shielded.
Even this approach is considerably less than perfect.

There could be other factors not considered.

.

[rabbit73]

Originally Posted by bobsgarage

I'm just wondering if my screenshots are showing in the post above? When I save the post they were all there

When I look at the text box for your post #2 as if I were going to quote you in a reply, I see links to attachments. Those links have the BB code to show the images in the post, but the images do not show. They do not show because you didn't add the images as attachments to the post. You must first add images as attachments to a post, then you can add the BB code for img so they will show in the post. Like this:





https://www.avsforum.com/forum/25-hd...l#post32920857

[bobsgarage]

Originally Posted by rabbit73

When I look at the text box for your post #2 as if I were going to quote you in a reply, I see links to attachments. Those links have the BB code to show the images in the post, but the images do not show. They do not show because you didn't add the images as attachments to the post. You must first add images as attachments to a post, then you can add the BB code for img so they will show in the post. Like this:

Rabbit,

Not sure what happened there. When I previewed my post, It looked great, all the pics were there, the links surrounded by: [IMG] [/IMG].


Then later when I looked, the images were gone. I have seen this before when closing the "manage attachments" box, I have lost my attachments. Possibly by uploading from my phone and editing it on my phone? Dunno.

I fixed it, for now.


Test pic:

[bobsgarage]

Originally Posted by tripelo

They?

See one image. It is open combiner case.

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


There could be other difficulties, but a noticable one is:

A difficulty with combining UHF with Q-wave matching is the union of the three cables.
All parameters are more critical at UHF, than VHF.

Inside the open case, the impedance is no longer 50 Ohms or 75 Ohms.
Therefore the correct impedance match of 37.5 Ohms (parallel combo)
to 75 Ohms via the Q-line is not occurring.

Since at VHF the dimensions (wavelength related) are about three times larger,
then the mismatch caused by impedance bump at the joint has less effect (although it is still there).

Best to join all cable as closely as possible (including shields).
Maybe could do that by drilling the case and running shields all in.
That means very short distances of cable that are not fully shielded.
Even this approach is considerably less than perfect.

There could be other factors not considered.

.

Hi Tripelo.
Well, I can try that, since this isn't working.

I figured the "T" connector would do all that is required:

[bobsgarage]

So back to my RG 8 stubs. I have been trimming back the shielding simply because I don't want it hanging out past the connector shell and I figure the inner barrel will make contact with it anyways. You are correct in the one photo I sent you I had trim back the outer layer too far. That's kind of an old pic I don't actually do it that way at least now. I usually trim back the outer skin to about 3/16".

Then I shove and I shove as hard as possible the connector all the way on, It Ain't Easy. Quite a pain actually. I've ruined many a connector by not putting enough effort into shoving them on as far as possible.

So I'm thinking that maybe I shouldn't be trimming back the shielding as far as I have been. Or maybe this RG 8 isn't what it says it is? I figured by right, all RG 8 must be 50 ohm...

I kind of liked my idea of adapting the RG 11 connectors to work with the RG 8 as it makes it so convenient to test. It seems like it should work.

So I'll ask this question. Do you think that soldering the tips from the RG 11 connectors on to the RG 8 changes something? What I'm wondering is is the cable length sacrificed because the contact tips add another inch at each end of the cable.

For example. If the overall stub should be 4 and 1/2 inches, the RG 8 has to be cut to 2.5" inches because the tips from the RG 11 connectors are 1" each.

I've got plenty of RG 8 and RG 11 connectors to play with. I'll make more.

[tripelo]

Originally Posted by bobsgarage

I figured the "T" connector would do all that is required:

Yes it might.

Have examined a few of those. Those examined here are not good quality.
The connection inside was pressure contact, probably very light pressure contact.

The parts that grab the insert connector pin were fragile, easily damaged by insertions.
And they had very little grip on the inserted connector pin.

Seem there was no design to match the impedance inside the 'T'.

Your 'T's' may be better quality.

Even with quality shortcomings, if you can insure good reliable contact,
the 'T' could be a somewhat better way than open loop connections.

Do you think that soldering the tips from the RG 11 connectors on to the RG 8 changes something? What I'm wondering is is the cable length sacrificed because the contact tips add another inch at each end of the cable.

For example. If the overall stub should be 4 and 1/2 inches, the RG 8 has to be cut to 2.5" inches because the tips from the RG 11 connectors are 1" each.

Depends on soldering technique.
Soldering connector tips is probably only a small small disturbance.

The length of the Q-line includes everything between the 75 Ohm line
and the point at where the joint is made.

.

[bobsgarage]

So, I ask the members here. If my 1/4 wave matching section isn't doing the trick, what should I do?

Do the 1/4 wave transformers have a very narrow band? Are they even practical?

I sure don't want to go to the trouble of combining two VHF or two UHF antennas only to lose some gain due to combiner loss.

What's the deal with the Ferrite beads? I'm sorry I just don't know these things..

[tripelo]

Originally Posted by bobsgarage

So, I ask the members here. If my 1/4 wave matching section isn't doing the trick,
what should I do?

Most antenna analysis considers the antenna is in free space.
That is far away from earth or any electromagnetic material (conductors in particular).

Stacking antennas close to earth (or things) without precise measurements
of the environment (beyond hobby capability) is unlikely to provide the
desired theoretical gain values. This is particularly true for vertical stacking.

For good results both antennas must see the same signal wavefront.
This is not likely with vertical stacking at common heights,
and may not happen with horizontal stacking.

The likelihood of dissimilar signal wavefronts for each antenna,
plus losses in the combiner, makes stacking antennas for increased gain
an uncertain proposition.

However, horizontal stacking often has the best chance for satisfactory results.

… Do the 1/4 wave transformers have a very narrow band?

In theory, 1/4 wave transformers are narrowband.
The term ‘narrowband’ probably means different things to different people.
But, most would consider the UHF TV band to be a wide band
(approaching 30%).

Quarter wave transformers match impedance perfectly at only one frequency.
At other frequencies the impedance match degrades with distance from the center.

Some may say that the loss (dB) of a quarter wave transformer is not very
significant over the UHF TV band. That usually is true.
However the impedance mismatch can be significant.
Impedance mismatch causes signal rejection (mismatch loss).

Most TV antennas and preamp/tuners have relative poor impedance control
(not 75 Ohms).
The performance of a mismatch quarter wave transformer working against
a mismatched preamp/antenna combination (without deeper considerations
and measurements) is often unpredictable

Nevertheless, a quarter wave transformer could provide better performance
compared to a ferrite-based splitter.
This is mainly because ferrite-based splitters have some inherent loss
(independent of mismatch) and some splitters also have appreciable
impedance mismatch.

Some of the difficulty of Quarter Wave transformers at UHF is the impedance variation
at the mechanical/electrical joint

…Are they even practical?

For most, achieving theoretical results probably is not practical,
but, ‘one may get close enough for practical purposes’.

…I sure don't want to go to the trouble of combining
two VHF or two UHF antennas only to lose some gain due to combiner loss.

Some combiner loss is unavoidable.
It is the amount of loss that is of most concern.

…What's the deal with the Ferrite beads?

Ferrite is often used to attenuate signal strength,
there are all kinds of beads available for that purpose.

Commercial baluns and many signal combiner/splittters use ferrite as a core
to help provide transformer action (inductance etc).
Manufacturers select particular kinds of ferrite that are less lossy and also
have capability to increase inductance etc.
The use of ferrite is a tradeoff.
Without the ferrite, the commercial baluns and splitters could be either larger
(if wound with air or other core) or more narrowband, or both.

.

[bobsgarage]

Thanks Tripelo.

Calaveras, a guy with some great technical knowledge like you guys here, told me his one quarter wave transformer does the job. He's moved it around from different arrays. He also told me it was fairly broadband across the spectrum.

His is soldered permanently. I could try that.

The reason I made my 50 ohm quarter wave stubs with the RG11 connectors is so I could easily substitute other lengths.

I have to either find out what I am doing wrong with my 1/4 wave sections or..
Do the 300 Ohm twin lead harness, which seems like a lot of trouble too.

I have done some research but photos and diagrams are hard to find.

My local electronics store Chester's Electronics in Kenosha Wisconsin had everything I need. So I bought everything to start. It was curbside pickup. They're being extra careful too. It was cool, the salesman came out talk to me and knew exactly what I wanted :



20 bucks. It's worth a try. I do not know the values of the ferrite.

[bobsgarage]

Had some successes after making an alignment tool for my 91XGs :







Here's some scans after perfectly aligning the antennas.. here you can see a comparison of the combined antennas versus the single antenna:

I always like to scan a single antenna versus the combined to start my sessions :




Comparing both 91xg antennas to each other:

]
,

[bobsgarage]

. The first screenshot is a comparison of two different lengths quarter wave sections. I have found that you can tune the frequencies a little bit by changing the length.



Here's some screenshots in these screenshots the superiority of the quarter wave stub is showing over the regular Holland combiner:









Here's a side-by-side comparison between both XGs. Tested singly. East vs West or left and right if you prefer. Single 91XG: