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Base Changing the "look" of an antenna

Greg T

WDX-945 (Jazz Singer) Upper Peninsula of Michigan
Sep 18, 2014
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Escanaba, Michigan
I have a question that may not have been asked before. At least, I can't find where someone has asked it. I have a Sigma Silver Rod 5/8 wave base antenna that works extremely well. But, the tuning favors the lower side of 11 meters and below. I don't really have an SWR problem, it's just that I can go way down out of the band without issue, but when I go up the SWR gets higher sooner. Without writing a novel here, I know I could shorten the antenna, but in my current physical condition this is not a choice. I sold all my equipment years ago so I don't have a tuner, but I was wondering if there was something I could do to make my antenna "look" shorter to my radio/amp. Well, the radio isn't the issue as it has a built in tuner, but I go up out of the band quite often for DX and don't want to lose any pills in the amp. Would adding capacitance across the coax have any effect?
 
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Check this thread...

https://www.worldwidedx.com/threads/resonating-coax.133171/

Now some cited the OP for using the word Resignate...which if you review kinda' spoiled the whole mess.

But the concept was to make a "tuning stub" to offset your SWR issues since you couldn't "fix" the problem due to physical constraints...

The idea here is not new, but explaining it can take VOLUMES of effort...

upload_2020-7-28_0-1-57.png
It's design...
A Wiki explanation...
upload_2020-7-28_0-3-3.png

Another type is called Balun...
upload_2020-7-28_0-13-27.png

Now, between these two - you may also appreciate how Amplifiers are made...only by folding the tubing, adding ferrtites and coupling the signal thru the use of winding within the assembly. You can take a complex Conjugate match mess and actually transfer energy from one low-impedance to a higher impedance - they call this a Balun, but too, a Balun is a form of transformation.

Now you have two choices, to use a Balun to offset the poor SWR problem - or try a Tunnig Stub.

Both have merits, but both will not fix the antenna and it's SWR issue, but using these to transform the poor SWR into an acceptable SWR for the Radio and Amp to function - these can work.
 
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And...on a separate note...

PDC-1089-01.png

IF you have an old TVI filter from CB days still laying around - put that in line, just after the amp - and check SWR at the radio...it may change - because the antenna being longer - being the problem, isn't going to change its' lengths or add capacitance, but the filter inside the TVI unit, can see there is a different impedance - so therefore present (exhibit?) a different impedance to make the SWR change to what it is (What you see) when you use it in line - past the amp.

I only hope that TVI filter is rated for the WATTAGE your amp will put thru it.
 
Handy Andy, thanx so much for the info. This is exactly what I need to tweak a bit. Easiest method first, of course, I will install the low pass filter and check the results. I am going to have a meter installed after the amp so I can manage the output and antenna match so this will be a simple test. As I mentioned, the input match to the amp in inconsequential as the radio has a built in tuner. A good match for the amp is critical. It may or may not be an issue as I have not yet put the amp inline. I thought I would have had it by now but it hasn't even been shipped yet. I noticed with the radio without using the tuner I can go to 27.705 and it just touches the 1.5:1 reading. This isn't even an issue with the radio, but as the power increases from the amp it could be. Hopefully the box gets here so I can begin. Thanks again for this excellent info!
 
I'm hoping that using a TVI filter - since they are "low-Pass" themselves may help lower the SWR to a tolerable level - if it goes the wrong way - well, then we know we'll have to use other options...
s-l1000.jpg

There also was a "tuning box" - many were made and even come as kits ...

But again, wattage ratings and the inability to fix the fixed tuning the antenna is - the options are there, but cannot guarantee results.

I'm hoping someone would chime in with a thought or two to add to this.
 
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There also was a "tuning box" - many were made and even come as kits ...

But again, wattage ratings and the inability to fix the fixed tuning the antenna is - the options are there, but cannot guarantee results.

I'm hoping someone would chime in with a thought or two to add to this.

The amp is going to be a Texas Star DX500V that will not be pushed hard. If I dial it up to, maybe, 400 pep watts or so I'll be happy. I have two of the low pass filters I can try. May be some subtle differences. One is an old cylindrical E.F. Johnson and the other is an 80s vintage Workman.
 
Gotta change the SO239s in the old Johnson low pass filter. Went back to check it again and it's at 1.6:1 . One end is pretty loose and the center insulator is cracked. Gotta drill them out and replace them. I knew it couldn't possibly be THAT easy!! LOL.

I did notice two variable caps on the ends of that filter. Possibly for matching, or do those tune the frequency?
 
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I did notice two variable caps on the ends of that filter. Possibly for matching, or do those tune the frequency?
Either or, you have nothing to lose to try them - just remember to "nail down" readings of the SWR and look for the orientation of the trimmers heads first - then see if it will help you "reload" the antenna.

This may affect FWD and REF settings in the SWR so you may want to just run the radio thru it first to see what it does in trying to work with the filter...

This is just a guide but taking pics to help remember where things were set is so you can return it back to the original as you found them if this doesn't work.

In light of this - this makes me think of the "Tune and LOAD" method of those older tube seta and FT-101

- man it's been a while...wish I had kept those notes...
 
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Either or, you have nothing to lose to try them - just remember to "nail down" readings of the SWR and look for the orientation of the trimmers heads first - then see if it will help you "reload" the antenna.

This may affect FWD and REF settings in the SWR so you may want to just run the radio thru it first to see what it does in trying to work with the filter...

This is just a guide but taking pics to help remember where things were set is so you can return it back to the original as you found them if this doesn't work.

In light of this - this makes me think of the "Tune and LOAD" method of those older tube seta and FT-101

- man it's been a while...wish I had kept those notes...
This is actually stranger than I thought. I don't have the amp yet so I have the radio connected to an external meter connected to the filter and then the antenna. Sometimes, about 50/50, I get conflicting readings between the meters. I'd like to get the combo "close" before inserting the amp because I won't be able to throw a carrier to check it the complete set up. My radio is much too hot for the amp on AM, which doesn't bother me because the amp is for SSB anyway. I guess the best way is to replace the SO-239s first, then check everything with the meter and attempt to adjust the filter, then insert the amp and try it on SSB. I know, not the most precise way of setting it up, but I need to make sure it's safe.
 
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Yes, might want to stabilize that rocket you're sending up - you don't want it tilting off and exploding once the "Launch" button is pushed.

Went looking for some info, so I took a guess...

Found this thread which mentions the "caps" and tuning for lowest SWR reflection - might be worth a read...
http://amfone.net/Amforum/index.php?topic=19940.0

But too, the thing if it's what I think it is, uses an anodized aluminum tube to house this all in, if it's oxidized, then there is part of your problem - trying to make good ELECTRICAL contacts with all parts thru the tubing.

upload_2020-7-29_23-17-38.png
This is the 35 one trimmer is in the photo just above the SO239

The opposite end...without the jumper...
upload_2020-7-29_23-19-55.png

Can you spare a pic to show us that filter - lots of members out here - one or two may know how to help you in aligning that filter.

In my efforts with "filtering" - there were several ways to "fix" goofy SWR but it never truly made the system any better - you were just "less worried" about the radio blowing it's finals. Hence the "Tuning Stub" methods. They helped when I was in an Apartment complex and needed to fix TVI problems. It's not a cure all, but if you post a pic of the Filter - you may have several "wave traps" in there for fine trimming or "notch filter" for specific TVI problems.

The filtering from back then was for NSTC stuff, the Analog days, with everything digital now, those methods are now moot - now they are more concerned with Out Of Band interference stuff.

Considering the design, the filter innards appear to be accessible - with would not be a bad idea to carefully back out the screws and see if you can use some alcohol wipes and even some elbow grease to burnish some of those connection points clean - you may need to "drill out" the SO-239's rivets and reinstall more dedicated bolt/nut stuff.

This is a labor of love - you have some work ahead of you but CB was never about simple forever - you start out simple - it gets more complex from there...
 
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Yes, might want to stabilize that rocket you're sending up - you don't want it tilting off and exploding once the "Launch" button is pushed.

Went looking for some info, so I took a guess...

Found this thread which mentions the "caps" and tuning for lowest SWR reflection - might be worth a read...
http://amfone.net/Amforum/index.php?topic=19940.0

But too, the thing if it's what I think it is, uses an anodized aluminum tube to house this all in, if it's oxidized, then there is part of your problem - trying to make goo contacts with all parts thru the tubing.

View attachment 39213
This is the 35 one trimmer is in the photo just above the SO239

The opposite end...without the jumper...
View attachment 39214

Can you spare a pic to show us that filter - lots of members out here - one or two may know how to help you in aligning that filter.

In my efforts with "filtering" - there were several ways to "fix" goofy SWR but it never truly made the system any better - you were just "less worried" about the radio blowing it's finals. Hence the "Tuning Stub" methods. They helped when I was in an Apartment complex and needed to fix TVI problems. It's not a cure all, but if you post a pic of the Filter - you may have several "wave traps" in there for fine trimming or "notch filter" for specific TVI problems.

The filtering from back then was for NSTC stuff, the Analog days, with everything digital now, those methods are now moot - now they are more concerned with Out Of Band interference stuff.

Considering the design, the filter innards appear to be accessible - with would not be a bad idea to carefully back out the screws and see if you can use some alcohol wipes and even some elbow grease to burnish some of those connection points clean - you may need to "drill out" the SO-239's rivets and reinstall more dedicated bolt/nut stuff.

This is a labor of love - you have some work ahead of you but CB was never about simple forever - you start out simple - it gets more complex from there...

That looks identical to mine. I will post a pic tomorrow afternoon. I have it apart already and it looks new-born in there. Everything is bright and shiny, even the solder joints. The entire filter consists of a few various coils (traps, I'm sure) bolted to, and insulated from spacers. These are situated so that each coil is in it's own little chamber. On each end is a small variable cap similar to this image. I am assuming these caps are to adjust for 50 ohm?

FWDIOE6FYTCOV8U.RECT2100.jpg
 
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Yes! Glad you found that!

Can't fix a mistuned old antenna, but can help you in taking that antenna from non-50 ohm into what appears as a 50 ohm load - but those caps also act like "wave traps" for Common mode issues - a different story...
 
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