Why would a specific radio cause a higher SWR reading?

[Lkaskel]

Hi Everyone,
I have an interesting condition I can seem to figure out. The user has a few radios that they use. When used they are connected to a Texas Star amp. with all but 1 radio the SWR's are 1.3 and they all have good audio. When they conect 1 radio (a Teaberry Stalker) the SWR's are over 2 and the audio is not distorted but does have a slight buzz or hiss with it. Everything is the same including coax jumpers and mics. The power on the Stalker is set to 4 watts AM and 12 watts SSB. The SWR's are high even on an AM carrier with no modulation. Why would a radio cause high SWR's?

 

[Shadetree Mechanic]

Maybe spurious emissions? Like part of the signal being transmitted is not on a frequency that the amplifier input is tuned to? Try putting an SWR meter between the radio and amp and see what the input match is like.

 

[9C1Driver]

I'm with Shadetree, bad match impedance match between the radio and amp.

 

[guitar_199]

I am a little out of my "experience level" here..... but......
I know that there is a filter between the final and the output jack. Could THAT be mistuned so that the radio is not quite set up for 50 ohm loads?

AND..... if you take that Teaberry and put it on another antenna... or a dummy load.... does IT also look funny there?

[edited to add]
if it looks good going into a known good 50 ohm load BUT maybe looks wonkey into other antennas as well..... then I am going to be on board with others.... that spurious 'off frequency' emissions are reacting with "reactances" in an antenna and causing more reflections than other radios.

 

[Lkaskel]

Thanks for each of your responses!! I agree with you on that thought but am still challenged with "what" is off frequency. I've aligned a ton of radios with the proper equipment and have never had this specific issue. It's warming up on the bench right now and I will go through it again. The one alignment challenge I have is the 54MHz trap as I got rid of my spectrium analyzer because it was soo large that it took too much space on the bench. I was thinking that this radio had some 2nd or 3rd harmoncis that was coming back to the radio as higher SWR's but I could not figure out why that would cause the weird audio issue. I did adjust L39 (the 54MHz trap) while throwing an AM carrier and it did cause the SWR's to rise but as it was adjusted it had the lowest possile SWR at a little over 2. The 2 coils before this trap are waxed and seem to have never been touched.

Do either of you have a though about which frequency I should focus on in this troubleshooting? I'll let you know how the re-alignment goes.

Thanks as always!!!

 

[Lkaskel]

And one last thought/question. What does the amp do to this scenario? Amps are not my gifting. You can connect any other radio to this setup and it's fine but for this radio. When I have the radio on the bench It seems to be ok. I'll verify that in a bit. The audio sounds fine coming through the bench radio. I am assuming that what ever is mis-aligned in the radio is being "amplified" with the amp.....

 

[TM86]

...I got rid of my spectrium analyzer because it was soo large that it took too much space on the bench.

Crappy but small and cheap replacement, the "tinySA": https://www2.randl.com/index.php?main_page=product_info&products_id=75243

Probably won't have everything you're used to, but it's slightly better than nothing.

 

[Lkaskel]

Crappy but small and cheap replacement, the "tinySA": https://www2.randl.com/index.php?main_page=product_info&products_id=75243

Probably won't have everything you're used to, but it's slightly better than nothing.

That's great!! At my tech level I was just using the one I had for a reference and not an accurate reading. In other words, I would see the 2nd and 3rd harmonic and adjust for the lowest level not for measuring to a certain level below the fundamental. Something like this could work fine for me I believe.

Maybe someday when I do not have a full time job and have more time to learn I can get another more accurate one.

Thanks again!!

 

[144inBama]

Over the years I've had a couple of radios I just could NOT drive into a amp for that very reason, it turned out one radio was incompatible with the amp. I had the amp tuned to the radio I would be using and it worked great, but any other radio would have retarded high SWR. I never took it to a "radio shop" as we didn't have any within a 100 miles so the amp was the easiest route since the builder was 25 minutes away. I kinda remembering him saying something like "I'm not a radio tech, but something is bad wrong with this one". Sounded great on the air barefoot, but as soon as you flipped that switch (tried a couple of his boxes too), it started humming, had high SWR, and the audio all but disappeared but had great signal. I think it was doing about 6 PEP. Come to think about it, he turned the power up and the SWR went down but everything else got worse.....anyway, changed the amp tune and she did great for a few years until I could afford a better radio, put it back to the original tune and the new radio was rocking too.