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Old 12th Jun 2021, 11:37 am   #1
Damo666
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Default President Lincoln SWR issue "only" when meter inline.

Hi guys,

Real strange issue this one, and something I spent a few hours exploring yesterday without success.

I recently installed a Sirio 1/2 wave vertical antenna a few days ago, and the SWR is under 1.1:1 > 1.3:1 for the range of frequencies I use. As you'll probably be aware, this is a good SWR range.

On all my CB rigs, the SWR is showing the aforementioned readings with the SWR/Power meter inline, however, with my President Lincoln Mk1, when I fit an inline SWR meter, the SWR goes sky high into the red.

Without a meter fitted inline, SWR readings correspond with readings taken on all other rigs. I know this, as the Lincoln has got an inbuilt SWR test function.

I've found that altering the length of the patch lead to the meter changes the SWR, and here are some approximate readings. Again, this only happens with the Lincoln; ~8" lead - 5.0:1, ~18" lead - 3.5:1, ~5ft lead - 2.5:1

Increasing the Lincoln RF output power also reduces the SWR considerably.

I could use the Lincoln without an external SWR/Power meter fitted so see "correct" readings, but I like to have a SWR/Power cross needle meter permanently inline.

By the way, I've done this test with 2 different SWR meters, and the results are still the same.

Can anybody please shed some light on this odd issue?
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Old 12th Jun 2021, 12:01 pm   #2
G0HZU_JMR
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Default Re: President Lincoln SWR issue "only" when meter inline.

It's possible the PA stage in the CB is going unstable when you present a certain antenna load to it and it may be producing a large out of band spurious signal. This signal would have to be able to get through the output LPF so it might be going unstable at a nearby frequency where your antenna has a very poor VSWR. As you change the coax length you change the impedance presented to the PA so this might affect how strongly it goes unstable.

CB amplifier stages are generally well behaved but I vaguely recall that the PA device is an MRF479 in those old radios and these have been obsolete for a long time. You may find that someone has fitted an alternative type of transistor (or a fake one) and it may be more prone to instability. I also vaguely remember some grounding wire link issues with these old President Lincoln radios.
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Old 12th Jun 2021, 1:15 pm   #3
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Default Re: President Lincoln SWR issue "only" when meter inline.

Quote:
Originally Posted by G0HZU_JMR View Post
It's possible the PA stage in the CB is going unstable when you present a certain antenna load to it and it may be producing a large out of band spurious signal. This signal would have to be able to get through the output LPF so it might be going unstable at a nearby frequency where your antenna has a very poor VSWR. As you change the coax length you change the impedance presented to the PA so this might affect how strongly it goes unstable.

CB amplifier stages are generally well behaved but I vaguely recall that the PA device is an MRF479 in those old radios and these have been obsolete for a long time. You may find that someone has fitted an alternative type of transistor (or a fake one) and it may be more prone to instability. I also vaguely remember some grounding wire link issues with these old President Lincoln radios.
Hello Jeremy,

This Lincoln is one of the last in the production line and uses an MRF455. I inspected it a while ago, and it appears untouched.

Thank you for the explanation, but the antenna has got a really low SWR over the range I'm using.

The problem I'm seeing is "only" when I fit an external SWR meter - but when I connect the antenna directly to the Lincoln (without a SWR meter), it behaves normally & the internal SWR function on the Lincoln shows this.

In a nutshell, the high SWR is only showing when I connect an SWR meter.

I hope I'm making sense.
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Old 12th Jun 2021, 1:28 pm   #4
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Default Re: President Lincoln SWR issue "only" when meter inline.

Are the instability issues still present when you replace the antenna with a dummy load ?

If so it must the combination or radio and meter. Only an analyser will indicate both Z and
resonance of an antenna.
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Old 12th Jun 2021, 1:50 pm   #5
Cruisin Marine
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Default Re: President Lincoln SWR issue "only" when meter inline.

I would suspect instability in the output stage.
If not you will have to narrow down the problem, but you will possibly need some test gear possibly to get anywhere.

If you have a spectrum analyser, then this will show up no problem.
If you don't have a SA, then you can always see instability on an oscilloscope, just measure along the supply line and you should see some weird things.
If you have neither, then monitoring off frequency is your last option to see where the sproggies are.
Transistors can go bad and cause this, but other components can too, check the decoupling capacitors, and add some more if need be- you can never have too much decoupling.
Quite often, instability can be caused by a mechanical problem like a bad connection on the antenna lead inside the set or a poor connection on the SO239. Check the grounding is solid and that the screws are done up fairly tightly too.
Look for aged soldering on the PCB too, and remove and re-flow if needed.
Another thing to look out for is corrosion, I have seen many an ingress of water in to the PA stage leave a nasty deposit- almost every time leading to a PA taking off.

Last edited by Cruisin Marine; 12th Jun 2021 at 1:59 pm.
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Old 12th Jun 2021, 3:12 pm   #6
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Default Re: President Lincoln SWR issue "only" when meter inline.

My first investigation would be to see what happens if you disconnect the antenna from the meter and put a 50-Ohm dummy-load on the output side of the SWR-meter.
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Old 13th Jun 2021, 6:24 pm   #7
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Smile Re: President Lincoln SWR issue "only" when meter inline.

Many thanks for the replies, guys, & please accept my apologies about the delay in replying; the kids here are very demanding & distract me.

In response to the question, I have used a dummy load & the SWR is absolutely fine.

SWR is also fine when the rig is connected directly to the antenna.

I too think it's spurious emissions, but I just can't get to grips with why it only happens when an SWR meter is inline?

I will nip the screws up internally & externally on the rig, just in case that's the problem.
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Old 14th Jun 2021, 6:59 am   #8
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Default Re: President Lincoln SWR issue "only" when meter inline.

Sticking with the Lincoln + external SWR meter + Aerial combination, try this:

Vary the length of the patch lead between the radio and the meter (if you have a selection of patch leads try longer ones and shorter ones).

Similarly, try adding some length to the aerial coax (use one of your patch leads plus a female-female S0239 barrel connector).

You need to change the length of the coax by a metre or more for this to have any appreciable effect.
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Old 14th Jun 2021, 12:01 pm   #9
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Default Re: President Lincoln SWR issue "only" when meter inline.

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Originally Posted by SiriusHardware View Post
Sticking with the Lincoln + external SWR meter + Aerial combination, try this:

Vary the length of the patch lead between the radio and the meter (if you have a selection of patch leads try longer ones and shorter ones).

Similarly, try adding some length to the aerial coax (use one of your patch leads plus a female-female S0239 barrel connector).

You need to change the length of the coax by a metre or more for this to have any appreciable effect.
Hi Sirius,

I have 3 different lengths of patch lead for between the rig & SWR meter, & these are the results. Notice the longer the patch lead, the lower the SWR; 8" lead - 5.0:1, ~18" lead - 3.5:1, ~5ft lead - 2.5:1

Turning RF power up also reduces the shown SWR, and when turned up to max' power (10W) the SWR is fine.

I thought I had a barrel connector for connecting 2 x PL259, but I can't find it. What you mention about increasing length is a good idea.

I'm seriously considering purchasing a decent 0-30 MHz LPF as this is looking like harmonics/spurious RF as has been mentioned.

What do you think about the installation of a low pass filter?

Last edited by Damo666; 14th Jun 2021 at 12:04 pm. Reason: Missing information
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Old 14th Jun 2021, 4:09 pm   #10
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Default Re: President Lincoln SWR issue "only" when meter inline.

Sounds like something funny happening on the feeder.

As the aerial is a new-install the positioning of the feeder relative to the aerial may be causing this. (Or other metallic objects nearby) Ideally it should come down vertically for about a half wavelength.

I would suggest trying an RF choke just below the feed point…..wind half a dozen turns of coax about 6” diameter and tape up, or wind on a plastic pipe etc.

A test for currents on the outer of coax is to grip the cable or coil it up while watching the reflected power. If it changes then you know you need to choke off those currents. Make sure the reflected power indicated is not way off the scale on the meter when doing this otherwise you might not see any changes.

The Lincoln’s PA might indeed be a bit touchy and the placing of the SWR meter inline might just be taking it over the edge. Can you try it on a friends aerial?
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Old 14th Jun 2021, 8:10 pm   #11
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Default Re: President Lincoln SWR issue "only" when meter inline.

I remember a number of historic instances where problems of unexpectedly bad SWR on half-wave vertical CB antennae were solved by substantially altering the length of the coax going to the aerial, hence my earlier suggestion to Damo.

However one thing which was always a given even on a working setup was that the higher the output power, the higher the indicated SWR even after the meter had been appropriately readjusted for FSD in the 'FWD' direction, so Damo's situation where turning the power UP makes the SWR BETTER is highly unusual, and lends weight to the idea that the radio may be going unstable and generating other frequencies (to which the aerial would present a bad SWR) when running at low power. Unfortunately the only thing which would show this properly would be a spectrum analyser, or possibly an SDR if anyone nearby has such a thing.

The question is, why does this only happen when the external meter is inline? I'm assuming that the bad SWR Damo sees is being indicated by the radio's onboard meter.

Damo, I think it's definitely worth (experimentally) changing the length of the coax on the aerial side if you can find a joiner.
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Old 15th Jun 2021, 1:47 am   #12
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Default Re: President Lincoln SWR issue "only" when meter inline.

@Sirius, Richard675

Thank you both for the tips, & also my gratitude to others for their kind assistance.

I've not got around to opening the Lincoln up, but one of the first things I'll be doing is nipping up all the PCB to chassis screws, then checking decoupling cap's. If this fails to remedy the problem, I'll then fit an open air choke just below the antenna feedpoint.

I've found a website that recommends the optimum amount of windings for various air wound/ferrite chokes, but some of this is beyond my low level of RF experience; I'm thinking around 20 turns of RG58, 4" diameter air wound unless I've misinterpreted the information on this site? http://www.karinya.net/g3txq/chokes/

I'm currently using 9mm RG213, & frankly, it's awful in terms of maneuverability. I've got about 3 metres of it spare on my setup, & it's difficult to hide here in the bedroom.

I'm going to invest in something a little thinner, perhaps good quality RG58 or something - then keep cutting around 1 metre at a time in the hope it sorts the problem. In the meantime, I'm going to cut about 2ft at a time from this RG213 and see if that improves things. I've got nothing to lose, as I've got about 3 metres redundant & in the way.

@Sirius - when the SWR meter is placed inline, both this meter & the rig SWR meter show exactly the same poor SWR.

I could use this rig connected directly to the antenna & forget all this ever happened, but I'm adamant that I now need to get this anomaly sorted out.

Last edited by Damo666; 15th Jun 2021 at 1:53 am.
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Old 15th Jun 2021, 7:06 am   #13
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Default Re: President Lincoln SWR issue "only" when meter inline.

Quote:
@Sirius - when the SWR meter is placed inline, both this meter & the rig SWR meter show exactly the same poor SWR.
This again suggests a possibility that the radio is going unstable so that a high percentage of its output is on frequencies to which the aerial presents a bad SWR. That's the thing about SWR / power meters, they don't really care, up to a point, what frequency the signal is. They will still show it.

I wouldn't normally swap out thick coax for thin coax - the reason you're using thick coax to begin with is because it is lower loss. It is admittedly harder to get it to go around corners and the radio can often end up where the coax wants to be, not where you want it to be.

A bad time to be having trouble with your radio. I'm in far NW Scotland at the moment and for a couple of hours yesterday I could hear UK stations from all the way across the south, including south Wales. Even managed to work a few (Midland 78, 4W FM).
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Old 15th Jun 2021, 7:16 am   #14
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Default Re: President Lincoln SWR issue "only" when meter inline.

Another question, what make / brand is the external SWR meter and what frequency / frequency range is it nominally meant for? Is it a CB specific item or is it a repurposed amateur unit?

Sorry to persist with this but it does seem odd that everything is OK until you put that meter inline - I know you did say that the radio + external meter behave OK when the aerial is replaced with a dummy load.
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Old 15th Jun 2021, 8:50 am   #15
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Default Re: President Lincoln SWR issue "only" when meter inline.

Quote:
Originally Posted by SiriusHardware View Post
Another question, what make / brand is the external SWR meter and what frequency / frequency range is it nominally meant for? Is it a CB specific item or is it a repurposed amateur unit?

Sorry to persist with this but it does seem odd that everything is OK until you put that meter inline - I know you did say that the radio + external meter behave OK when the aerial is replaced with a dummy load.
I've used 2 different SWR/power meters as my first thought was that one might have been defective; I used a KPO standard 27MHz CB meter, and also my trusty Avair av20 that covers 1.6 - 150 MHz.

I've tested both of these on other rigs & they're in good working order.

There's no need to apologise about the questions, as I really do appreciate you taking time out to get to the root of this problem.

The SWR issue "only" when the external meter is installed is certainly very strange to say the least, and that's the thing I just can't get to grips with.
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Old 20th Jun 2021, 9:58 pm   #16
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Default Re: President Lincoln SWR issue "only" when meter inline.

Damo, did you try adding some more length to the coax at the aerial side of the equation, or failing that, taking some length off? It may have a more dramatic effect if you try it on the coax to the aerial instead of on the patch lead between the radio and SWR meter - although that was worth trying and did cause some change in the SWR.

Here's a thought: You say you don't (or didn't) have any female-female connectors but you do have at least two SWR meters and a selection of patch leads. So what happens if you daisy-chain two of the meters in series (in effect, using the second one as a cable joiner).
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Old 20th Jun 2021, 11:24 pm   #17
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Default Re: President Lincoln SWR issue "only" when meter inline.

Damo,
One other suggestion if I may.
If you have no other means to find out if the rig is unstable, try tuning another receiver anywhere on an HF frequency around 10 MHz- 20 MHz and listen to see if you can hear a "whoosh" as a carrier sweeps past when you drop key/ release the PTT on the mic.
This is another rough indication of an unstable PA stage, as the PA oscillation will sweep as you remove the drive and it will probably go potty, this may work even if the PA supply is switched too.
This is a rough and dirty way of doing things, but, a Spectrum Analyser is the best way to see the truth of course.
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Old 25th Jun 2021, 9:05 am   #18
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Default Re: President Lincoln SWR issue "only" when meter inline.

Hi guys,

I'm away from home until early next week, but I'll get around to doing as mentioned upon my return.

I purchased 15m of RG8 mini & a couple of PL259 barrel connectors so I'm now good to go.
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Old 25th Jun 2021, 11:57 am   #19
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Default Re: President Lincoln SWR issue "only" when meter inline.

I hope those were S0239 (Female) and not PL259 (Male) barrel connectors you ordered.
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Old 25th Jun 2021, 8:35 pm   #20
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Default Re: President Lincoln SWR issue "only" when meter inline.

I'd go for SO239 father than S0239 myself
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