Tom
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.
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.
Jerry, K4SAV
K4SAV wrote:
>I hope you mean: center fed zepp = dipole.
>
>It's impossible to exactly answer your question as stated, but here are
>some things to consider.
>
>If you are feeding that 80M dipole with coax, you are going to see huge
>power losses in the coax on 160M. The SWR will be >100:1 (don't know
>exact value, but it will be huge). To give you a similar example, I had
>a friend who was using a 40 meter dipole fed with coax on 80M. When he
>finally went to a real 80M dipole, his signal went from S3 to S9. (Don't
>rely on these exact numbers, but the difference will be large.)
>
>I wouldn't recommend just coiling up a bunch of wire as you stated. You
>will be making a big loading coil, the inductance of which is totally
>unknown. The antenna will probably be resonant somewhere in the
>broadcast band if you wind a coil with 50 feet of wire.
>
>If you are using ladder line for the feedline, the transmission line
>losses will be a lot less on 160, but your tuner will probably not like
>it. However, in this case the impedance seen by your tuner will be a
>function of feedline length, and you may be able to vary the feedline
>length and find something your tuner can match. This isn't really a
>very good approach if you use high power.
>
>You could convert your dipole to a trap dipole and make it resonant on
>both 160 and 80. On big disadvantage of doing this is that the
>bandwidth will be much narrower (on both bands) than that of a dipole
>without the trap. Bandwidth on 80 and 160 can be a big problem.
>
>If you can put in ground radials, you could tie both sides of the
>feedline together, bring the feedline straight down, and feed it as a
>top loaded vertical (a tee) on 160. Don't try this without the radials!
>You will also need a matching network at the base where the vertical is
>fed and the radials are tied. This is a significant amount of work, but
>it will make a very respectable antenna on 160, especially since you
>have an 80 ft vertical section. An 80M dipole configured as a tee with
>an 80 ft vertical section will probably be resonant around 1.4 MHz, but
>at 1.8 MHZ the resistive part plus ground losses will be somewhere
>around 45 ohms, which means for the matching network you may be able to
>get away with a single series capacitor.
>
>These are a couple of suggestions, there are many other possibilities.
>Start looking in antenna books for ideas, and don't just throw something
>together.
>
>Jerry, K4SAV
>
>Tom Osborne wrote:
>
>
>
>>HI Guys
>>
>>In working the NAQP last night, I had quite a few requests to QSY to 160 from
>>80. On 80 I am using a ~130 foot center fed zepp. Going to 160 only
>>requires a quick re-tune of the tuner so that's no problem.
>>
>>Question is, if I were to add to the ends of the antenna 50 feet of wire in a
>>coil, and have a 10 foot pigtail, would that improve the operations on 160,
>>and would it make any difference on 80? It is up about 80 feet in a tree so
>>that's not really very high on 160, but would like something a little
>>better. 73
>>
>>Tom W7WHY
>>_______________________________________________
>>
>>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
>>questions and ask for Sherman, W2FLA.
>>
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>>
>>
>>
>>
>>
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
>questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
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