Quote:
Originally Posted by G0HZU_JMR
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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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.