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54Mhz Slug Suprise


Sr. Member
Aug 3, 2011
So I installed a 108" whip and some 18' of LMR 400. I had a Uniden PC78XL that I bought about 10 years ago as NOS.

It looked like NOS in every single way right down to none of the hardware, power cable, mic looking like new.

The good news the antenna is as flat as a slab of finely machined granite.

Odd it dead keys 4 watts and swings to 8 avg. pep 10 watts. It can be heard on 107.9 on my cars factory radio. Only on 107.9 and it flanks out the in dash radio.

All my gear is up in the attick. Home reapirs THe roof was off last summer.

So I have not taken the tins off yet. It strikes me as odd that this would be a hack job given the low wattage and lack of swing. Even a golden screw driver with light bulb type would not set the carrier at 4w swinging to 8-10w.

Also the internal SWR bridge was reading 2-3 across 40 but both of my external meters read flat across the band.

Also I have a son moving home from college in March until he figures out if he is joining the work force or going back for his master. His room is was my work are for both reloading and electronics.

So soon I will take the tins off and see if I see any signs of butchery. Not used to working on primitive vintage CB gear. LOL Great stuff thogh. I just graped it off the shelf and opened the NOS box because it was convient. I had only just installed the antenna and it was dark and late around 11pm. I knew I could just plug it into the cigerette lighter/power adapter. I have not run power wires from the battery yet. I am glad I pulled it off the shelf and tried it though because I thought assumed it was in good working order.

Last time a radio was in this 21 year old car was probably 15 years ago and it had a Wilson 1000 magnet mount and TRC 453 with NPC mod and zero issues. I was young and stupid and had to try NPC with zero issues so this NOS PC78XL (suspect) must be really bad on harmonics for it to lock up AM/FM car radio and be heard on 107.9.

So I will try another radio in this application to make sure it is not the OEM AM/FM radio or my antenna system.

I love the performance of the 108" whip so far. I will test the performance of it more. I was suprised it was flat on CB since it is mounted to the trunk lid. I always had great results with 102" and 6 inch ext. bar on a bumper back when they where steel.

If all turns out well with the antenna system I will either put a Yeti Optima MKIII in it or a old Lincoln just depends on band width.

I will say this the DSC function on the Uniden PC78XL worked fantastic! I was suprised because the auto Squlech on the President Lincoln never worked that great. I might try a gently used Uniden Grant LT next since I can use it with the cigerette lighter/12 power port.

I will keep you guys abreast of the slightly boring and mundane adventure!

Per the title, I assume you are thinking this harmonic issue is related to the 54MHz trap. I assure you that it is not. The 54MHz trap is only for 54MHz (the second harmonic). I have adjusted a good number of those while watching a SA and it has absolutely no influence on the 4th harmonic (26.965*4=107.86MHz) or any other harmonics (other than the 2nd, of course). That's what the low pass filter does. What will produce a 4th harmonic is if someone spread the coils of that filter or there is distortion in the waveform due to misalignment.

FM car stereos have decent sensitivity, so a CB radio that close (even if technically in spec) would almost certainly wipe out the station when right on a harmonic like that. But from the sound of it, I think it needs a proper alignment anyhow with the swing not doing its thing, so you may as well plan on going through the whole radio.
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