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If I had a good transmatch and wanted to use a spare CB antenna on the amateur HF bands, would I be better off bypassing its tuning coil completely?

One ham pastime is using an antenna tuner to match a random conductor (house gutters, bedsprings, chain link fence, a head of lettuce…) and use it as a makeshift antenna on ± any band.

But in the case of a too-short monopole element that's been baseloaded to make it resonant at a lower than natural frequency — does that make it even narrower bandwidth and harder to retune than the original whip? Or will the antenna's coil simply work in concert with the capacitors and inductors in my tuner to force a match just as well as without the coil?

Does it make much a difference if I'm trying to electrically re-tune/re-match a loaded antenna designed for 11-meter on a close (10/12-meter) versus completely random (say 75-meter) band?

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There's a large amount of "it depends". There's nothing inherently bad about having both loading at the antenna and a tuner in the shack.

The antenna coil probably makes the antenna a better match for the coax, and therefore reduces coax loss related to high SWR, but that's not guaranteed, especially if you're far from the design band. The antenna impedance could be something far-out and you can't be sure unless you measure it. (Also, what seems to be a base loading coil might actually be an autotransformer!)

The antenna coil might make the impedance seen at the shack end of the feedline one that your tuner can deal with more easily / more efficiently, but it might not, too, because every feedline is an impedance transformer, and depending on the length of the coax, you might get unlucky.

Even if the impedance works out in your favor, the coil in a mobile antenna might be a fairly low-Q (lossy) one, and the tuner loss plus the mismatch loss in the cable from letting the tuner do all the work might be lower than the coil loss in the antenna. Or it might be the opposite.

The results depend on the antenna design, the coax quality, the coax length, the tuner build, and the frequency you're using — every case is different! You'll have to measure and experiment. Or just use the thing, you'll probably get out in any case.

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The loading coil would help by reducing the amount of inductance required from your tuner. If you were to remove it, the tuner would have to make up for that missing inductance by adding more of it's own, and potentially run out of inductance to give.

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  • $\begingroup$ Makes sense in retrospect! So I guess the only potential problem would be the case of trying to tune a longer antenna on a shorter band, in which case the antenna's built in inductor coil would be fighting the available capacitance of the tuner. Which I'm speculating wouldn't be a problem going from say 11m to 10m anyway, maybe a concern going to 6m… and by the time I'm any further into VHF then I probably have more convenient antenna options anyway :-) $\endgroup$ Commented Apr 5, 2022 at 20:25
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Adding devices in series to your antenna will add insertion loss. That is a fact, and there is nothing you can do about it, except removing everything that is not mandatory.

If your antenna is correctly adapted, remove everything which is not mandatory, including your antenna tuner. The antenna tuner is meant for a mismatched antenna which is not your case.

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Generally the less coils or inductors in an antenna that you can get away with the better. Every loading coil or inductor used for loading or matching reduces the bandwidth and efficiency of the system. In this case, also the loading coil will act as a low pass filter and effectively disconnect the part of the antenna above the coil on all bands below 27 MHZ. The coil is probably part of the antenna structure so i would short it out and then use the antenna matcher only to adjust the SWR.

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