Tuning a pre-tuned antenna

[WSBV579]

I've got a Nagoya UT-72 dual band antenna on my Jeep. I've just cut it down to tune for GMRS. I've got Ch19 (my main simplex channel) from 1.75 swr down to 1.42. I did get it to 1.19, trimmed a touch more to improve repeater channels, and ch19 went up to 1.42...so I assume I'm done cutting at that point. That leaves the repeater channels around 1.98...it was 3.10 before I started trimming, yikes!

First question...are those acceptable ranges, 1.42 simplex, 1.98 repeater?

Second question concerns my base antenna. I've got a Tram 1477, pre-tuned dual band. I checked the swr on ch19 and it's at 2.18. and repeater channels are around 1.50. Are those acceptable ranges, or should I try to figure out how to tune it...and if so, do I trim or lengthen?

Last question. Other than trimming/lengthening the antenna, is there anything else I can do to improve my swr?

[SteveShannon]

6 minutes ago, WSBV579 said:

First question...are those acceptable ranges, 1.42 simplex, 1.98 repeater?

They are, but some people would try to tune them. I think I would not. 

[SteveShannon]

1 minute ago, WSBV579 said:

Second question concerns my base antenna. I've got a Tram 1477, pre-tuned dual band. I checked the swr on ch19 and it's at 2.18. and repeater channels are around 1.50. Are those acceptable ranges, or should I try to figure out how to tune it...and if so, do I trim or lengthen?

Trimming would make the 462 MHz channels higher SWR because it’s already higher SWR than the higher frequencies.
Without sweeping it using an antenna analyzer it’s difficult to know which way to go for the repeater channels: is the bottom of the dip left or right of the repeater channel you looked at?.  

[WSBV579]

10 minutes ago, Sshannon said:

Trimming would make the 462 MHz channels higher SWR because it’s already higher SWR than the higher frequencies.
Without sweeping it using an antenna analyzer it’s difficult to know which way to go for the repeater channels: is the bottom of the dip left or right of the repeater channel you looked at?.  

Going to 470 MHz, the swr got even better. So I assume I would need to lengthen to make my ch19 better. I'm taking the antenna down on Monday to add a 10ft extension, I'll look and see if lengthening is an option.

As it sits, is 2.18 on ch19 and 1.50 on repeater channels acceptable? If so, I may not even bother tuning any further.

[SteveShannon]

3 hours ago, WSBV579 said:

Going to 470 MHz, the swr got even better. So I assume I would need to lengthen to make my ch19 better. I'm taking the antenna down on Monday to add a 10ft extension, I'll look and see if lengthening is an option.

As it sits, is 2.18 on ch19 and 1.50 on repeater channels acceptable? If so, I may not even bother tuning any further.

2.18:1 is acceptable, but not great; I’ve used worse. 

1.50:1 is just fine. I probably would not do much. 
If 470 MHz is even better SWR then the element is slightly too short. 

[WSBV579]

13 minutes ago, Sshannon said:

2.18:1 is acceptable, but not great; I’ve used worse. 

1.50:1 is just fine. I probably would not do much. 
If 470 MHz is even better SWR then the element is slightly too short. 

Alright, if I can figure out how to lengthen it...like loosen a set screw or something...then I'll see if I can improve it. If not, I'll just leave it as it. 

Appreciate the help.

[FreqCarl]

On 4/26/2024 at 10:55 PM, WSBV579 said:

Alright, if I can figure out how to lengthen it...like loosen a set screw or something...then I'll see if I can improve it.

Just setting up a GMRS base and purchased a Tram 1477 from a friend of mine.  I'm running around 90' of LMR400 to it and SWR's on all GMRS channels were a little over 2:1.  Probably ok, but I wanted to take the 1477 apart to see if I couldn't get it a little lower, but couldn't find any info on how to disassemble it. 

The setscrew on the base that holds the element inside the antenna is covered with a glue that I scraped out as much as I could, and gave it a couple seconds of a propane torch to soften up the glue.  That set screw is a 3mm Allen, and the other setscrews on the element takes a 2mm Allen wrench.  Once that setscrew on the base is backed out enough, the whole element pulls out.  There are 3 pieces of foam on the element, covering a coil, a union (or another coil, not sure), and another piece of foam on the upper portion.  This foam snugs up the element in the fiberglass/plastic piece, but will pull out.

I first tried extending the element by using some heat shrink over the tip and a piece of 3/32 welding rod.  SWR's were higher on both ch1 & 22.  I then took some foam off of the union to loosen the 2mm set screw and removed the top portion of the element and experimented with different length's of welding rod.  I have the original uncut top piece to put back on just in case.  

I found the lowest swr readings from a 120mm/4.75" piece of welding rod.  The original SWR readings on ch1/ch22 were 2.14/2.26, and with the shorter top element they are 1.28/1.82.  Haven't checked 467.xxx yet.

I'm not a seasoned ham or gmrs user, but messed with CB's 50 years ago.  So take my findings with a grain of salt, but this is how I disassembled my 1477 if anyone was wondering.

 

 

 

[Socalgmrs]

If a pre tuned antenna is any higher than 1.8 I send it back.  I just buy 712efc antennas and sent it. 1.5max in any gmrs frequency and 10’ of 9db gain.  200miles from a 20w.  Why any one would put up with a tram at 2:1 is beyond be.  

[FreqCarl]

1 hour ago, WRXP381 said:

If a pre tuned antenna is any higher than 1.8 I send it back.  I just buy 712efc antennas and sent it. 1.5max in any gmrs frequency and 10’ of 9db gain.  200miles from a 20w.  Why any one would put up with a tram at 2:1 is beyond be.  

The Tram 1477 is a pre tuned antenna, just not for GMRS.

[WSBV579]

11 hours ago, FreqCarl said:

Just setting up a GMRS base and purchased a Tram 1477 from a friend of mine.  I'm running around 90' of LMR400 to it and SWR's on all GMRS channels were a little over 2:1.  Probably ok, but I wanted to take the 1477 apart to see if I couldn't get it a little lower, but couldn't find any info on how to disassemble it. 

The setscrew on the base that holds the element inside the antenna is covered with a glue that I scraped out as much as I could, and gave it a couple seconds of a propane torch to soften up the glue.  That set screw is a 3mm Allen, and the other setscrews on the element takes a 2mm Allen wrench.  Once that setscrew on the base is backed out enough, the whole element pulls out.  There are 3 pieces of foam on the element, covering a coil, a union (or another coil, not sure), and another piece of foam on the upper portion.  This foam snugs up the element in the fiberglass/plastic piece, but will pull out.

I first tried extending the element by using some heat shrink over the tip and a piece of 3/32 welding rod.  SWR's were higher on both ch1 & 22.  I then took some foam off of the union to loosen the 2mm set screw and removed the top portion of the element and experimented with different length's of welding rod.  I have the original uncut top piece to put back on just in case.  

I found the lowest swr readings from a 120mm/4.75" piece of welding rod.  The original SWR readings on ch1/ch22 were 2.14/2.26, and with the shorter top element they are 1.28/1.82.  Haven't checked 467.xxx yet.

I'm not a seasoned ham or gmrs user, but messed with CB's 50 years ago.  So take my findings with a grain of salt, but this is how I disassembled my 1477 if anyone was wondering.

 

 

 

Thanks for messaging me about your comment. I don't have welding rod to experiment with, so I'll have to try trimming. Hopefully I can get similar results.

I've got time today, if I can get the mast down. It's a two person job...but I think I can manage.

[back4more70]

This sounds like a lot of work for a $35 antenna 

[WSBV579]

15 hours ago, FreqCarl said:

 

I've been trimming now, millimeter by millimeter, and I'm at 120mm-ish...and my swr is at 3.80 across basically all channels.

Never got the swr below 2.5 the entire time I was trimming.

[SteveShannon]

15 minutes ago, WSBV579 said:

I've been trimming now, millimeter by millimeter, and I'm at 120mm-ish...and my swr is at 3.80 across basically all channels.

Never got the swr below 2.5 the entire time I was trimming.

Are you trimming above the center coil or below?  I suspect the lower element was the UHF element, the center coil is a trap, and both elements combine to form the VHF element.

[WSBV579]

4 minutes ago, Sshannon said:

Are you trimming above the center coil or below?  I suspect the lower element was the UHF element, the center coil is a trap, and both elements combine to form the VHF element.

Above the coil. 467.000 is at 1.58, 462.000 is at 3.00.

Should I have been trimming below the coil? How bad have I f'ed up here?

[SteveShannon]

1 minute ago, WSBV579 said:

Above the coil. 467.000 is at 1.58, 462.000 is at 3.00.

Should I have been trimming below the coil? How bad have I f'ed up here?

Not badly. You’ve only cut off some metal.  You can always replace it.

Is the center coil (which I think is a trap) easy to remove from the lower element?

Do you want this antenna to be usable on two different bands or just GMRS?

If the answers are yes and just GMRS, then take the upper element and trap coil off and see what the bottom element measures for SWR on 433 MHz.  If it’s good, then you can start tuning it for 462-467.  You shouldn’t have to remove much and maybe you can test it with the pieces left over from the top.

[WSBV579]

1 minute ago, Sshannon said:

Not badly. You’ve only cut off some metal.  You can always replace it.

Is the center coil (which I think is a trap) easy to remove from the lower element?

Do you want this antenna to be usable on two different bands or just GMRS?

If the answers are yes and just GMRS, then take the upper element and trap coil off and see what the bottom element measures for SWR on 433 MHz.  If it’s good, then you can start tuning it for 462-467.  You shouldn’t have to remove much and maybe you can test it with the pieces left over from the top.

I don't really have a need for dual band anymore. I was gonna get my ham ticket, but GMRS suit my purpose just fine.

So, what your saying is, I can remove the could completely? How will that harm the dbi? 

[SteveShannon]

17 minutes ago, WSBV579 said:

I don't really have a need for dual band anymore. I was gonna get my ham ticket, but GMRS suit my purpose just fine.

So, what your saying is, I can remove the could completely? How will that harm the dbi? 

I don’t know how it will affect the gain. The saying with antennas is “Everything affects everything.” 
Do you have an antenna analyzer?

I wouldn’t start hacking away based on my suspicions.  But if you can take things apart in a way that allows you to put them back together easily, then feel free to play around.

[WSBV579]

Well, this has all been a lovely learning experience and everything...but I'm just gonna have to be without my base station radio until I can get a new antenna.

I've got a problem with not being able to leave "well enough" alone. I'm the type that wants to always try to fix it to make it better, whatever "it" may be. That didn't work this time.

Now that I've decided on GMRS, rather than getting my ham ticket as well, I guess I'll be buying a GMRS antenna when I can afford to.

[SteveShannon]

5 minutes ago, WSBV579 said:

I've got a problem with not being able to leave "well enough" alone. I'm the type that wants to always try to fix it to make it better, whatever "it" may be. That didn't work this time.

I have the same trait. Almost all engineers do. Usually it’s fun. Sometimes it’s frustrating. If you decide you want to try to make the pieces into a GMRS antenna I’ll do more research. 
I wouldn’t discard the pieces. Some ham would enjoy playing with them. 

[SteveShannon]

Here’s a video that explains how trap vertical antennas work:

 

[WSBV579]

17 minutes ago, Sshannon said:

I have the same trait. Almost all engineers do. Usually it’s fun. Sometimes it’s frustrating. If you decide you want to try to make the pieces into a GMRS antenna I’ll do more research. 
I wouldn’t discard the pieces. Some ham would enjoy playing with them. 

I'll be holding onto this antenna. I may decide to play with it at some point (maybe an antenna for our camping trips), but for now the frustration has hit me pretty hard and I'm kinda done with it for now.

I'm a mechanical/manufacturing engineer as well. Not leaving "well enough" alone is a bad habit of mine, though it typically does well for me in my career field. But radios, antennas, RF, SWR, dbi...it's all new to me and I'm still learning. 

I appreciate the help.

[tcp2525]

16 hours ago, WRXP381 said:

If a pre tuned antenna is any higher than 1.8 I send it back.  I just buy 712efc antennas and sent it. 1.5max in any gmrs frequency and 10’ of 9db gain.  200miles from a 20w.  Why any one would put up with a tram at 2:1 is beyond be.  

Sounds like they had a bad batch! I had to send the one I bought back as well. I'm waiting for anther production run to come in before I buy another. I take it Comet no londer has their antennas manufactured in the HungLo province of China?

[FreqCarl]

2 hours ago, WSBV579 said:

Above the coil. 467.000 is at 1.58, 462.000 is at 3.00.

That's odd.  The swr's on the 1477 I have are higher on the higher frequencies.  I was checking swr's with both my Surecom and a NanoVNA which were very similar.

The original top piece on mine is 7 15/16", 202mm if you want to go back.