Good reference Tom, and I notice it is your write-up. Nicely done.
I didn't want to recommend the approach of adding coils into the
antenna, because the 80M dipole is just too short, and this approach is
not nearly as good as feeding it as a vertical tee. But to give him a
direct answer to his question, I just did a comparison without feedline
or tuner losses, which did not justify the effort involved. Actually if
I had included these losses, it would have made the approach of adding
coils look better, because the feedline and tuner losses would have
decreased, compared to using the plain 80M dipole. (as you noted in your
latter comment). So maybe this comparison wasn't exactly fair, but
still, I don't think this is good way to do it, system efficiency will
still be low, and the 250uH coil is probably a disaster (didn't
investigate that one, other than the fact that at 1500 watts the voltage
across the coil in in the range of 15,000 volts).
I could not find a solution using an inductor across the feedpoint. The
size of the inductor can be reduced by moving it further toward the
center of the antenna, or adding more length to the pigtail, but this
totally messes up the impedance at 80M. So now we are back into the
trap approach which is not good either.
There may be some other possibilities using some kind of linear loading,
but this is now becoming a different antenna.
Jerry, K4SAV
Tom Rauch wrote:
>>To answer your question a little more directly, I made a
>>
>>
>few
>
>
>>simulations. I considered the configuration you
>>
>>
>mentioned, loading
>
>
>>coils at the end of the 80M dipole, and 10 ft sections
>>
>>
>hanging down, and
>
>
>>assuming not winding an arbitrary coil, but one which
>>
>>
>resonates the
>
>
>>antenna on 160M: Also assuming negligible feedline loss
>>
>>
>on both bands.
>
>The last sentence is the rub.
>
>Besides tuner loss issues feedline loss is significant on
>160 for a short non-resonant antenna.
>
>http://www.w8ji.com/short_dipoles_and_problems.htm has
>loss data for an antenna 70% of the full size length and no
>reactance compensation at the antenna. As we continue to
>shorten the antenna (without reactance compensation at the
>antenna) loss increases rapidly with small length
>reductions.
>
>
>
>>You will need about a 250uH coil - hard to build and
>>
>>
>large.
>
>
>>The resonance on 160M will be very sharp.
>>The impedance on 80M will no longer be resonant, but
>>
>>
>manageable with a
>
>
>>tuner and 300 ohm balanced feedline. (Which you said you
>>
>>
>were using)
>
>
>>You will pick up only about 1 dB gain on 160M due to some
>>
>>
>linearization
>
>
>>of the current along the 130 ft wire.
>>You will lose about 0.5 dB on 80M.
>>Doesn't sound like it is worth the effort.
>>
>>
>
>It may well be worth it when you consider feedline and tuner
>losses. IMO the best solution is the suggestion to tie the
>feeder wires together and feed the antenna like a "T"
>against a good ground. A second choice might be to put
>reactance compensation (the inductor) directly across the
>feedpoint. The inductor would have minimal effect on higher
>bands and require less inductance. That might be a solution
>as long as it didn't make the feedpoint resistance too high
>on 160.
>
>73 Tom
>
>
>
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