Hi Antenna guru's....

Can someone with the NEC or equal antenna simulator tell me what the impedance
of a dipole tuned for 28.4MHz is at 21, 21.25 and 21.450?

Also to other way around, what is the impedance of a 15m dipole cut for 21.25
as seen by a generator at 28.2, 28.5 and 29.0.

If I take this 10 meter dipole and attach a RG-58 cable to it, the
transformation of the reflected impedance to the sending end will vary with the
length of the cable and repeat every half-wave interval (take in account the
velocity factor of the RG-58 as 0.6).

What lenght of RG-58 should I use attached to this 10meter dipole to create the
highest SWR possible on 21.25 MHz?

My thoughts here is to have two dipoles attached to one feed, the first antenna
that the signal meets is the 15meter dipole. The second antenna will be
attached after another run of RG-58.

I hope to select the lenght of the 58 run between the two dipoles such that the
impedance reflected into the center of the 15 meter dipole is very high at
21.250.

I would hope to reach the same using the 10 meter band such that the 15 meter
dipole would present a high impedance to the 10 meter signal.

De KJ4UO

PDRUNEN wrote:
What lenght of RG-58 should I use attached to this 10meter dipole to create the
highest SWR possible on 21.25 MHz?

Any length will result in the same SWR.

My thoughts here is to have two dipoles attached to one feed, the first antenna
that the signal meets is the 15meter dipole. The second antenna will be
attached after another run of RG-58.

The feedpoint impedances of the 15m dipole and 10m dipole are different
enough, you don't have to worry about much interaction. With them in
parallel at the same feedpoint, the high feedpoint impedance on the 10m
dipole for a 15m signal will force most of the energy into the 15m dipole.
Likewise, the high feedpoint impedance on the 15m dipole for a 10m signal
will force most of the energy into the 10m dipole.

On 21.4 MHz, the 10m dipole will have a feedpoint impedance of 200+j450

On 28.4 MHz, the 15m dipole will have a feedpoint impedance of 29-j384

So just parallel them, tune 'em up, and enjoy.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:
On 21.4 MHz, the 10m dipole will have a feedpoint impedance of 200+j450

On 28.4 MHz, the 15m dipole will have a feedpoint impedance of 29-j384

Well, I obviously got these impedances reversed. Should have been:

On 28.4 MHz, the 15m dipole will have a feedpoint impedance of 200+j450

On 21.4 MHz, the 10m dipole will have a feedpoint impedance of 29-j384

Thanks, Jerry, for catching my mistake. But the paralleling advice stands.
--
73, Cecil http://www.qsl.net/w5dxp



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I have tried various combinations of multiple dipoles fed from a common
point, and they all worked. At resonance, the feedpoint impedance will
actually be closer to 50 Ohms than for a single dipole.

Note from Cecil's numbers that the other dipole add reactance. You will have
to compensate for that by adjusting the lengths of the dipoles. Start with
the lower frequency dipole first, but since 10 & 15 are rather close, you
might have to go back and forth a few times.

The main thing is to get separation between the ends. Having the two dipoles
at right angles would be optimum, but 15 degrees, or so, will work.

If you use Cecil's numbers to calculate the feedpoint impedances, don't
forget to convert everything to a parallel equivalence first.

Tam/WB2TT
"Cecil Moore" wrote in message
...
Cecil Moore wrote:
On 21.4 MHz, the 10m dipole will have a feedpoint impedance of 200+j450

On 28.4 MHz, the 15m dipole will have a feedpoint impedance of 29-j384

Well, I obviously got these impedances reversed. Should have been:

On 28.4 MHz, the 15m dipole will have a feedpoint impedance of 200+j450

On 21.4 MHz, the 10m dipole will have a feedpoint impedance of 29-j384

Thanks, Jerry, for catching my mistake. But the paralleling advice stands.
--
73, Cecil http://www.qsl.net/w5dxp



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It might be simpler to use open wire feedline and a link coupled tuner or tuner
with a balun in the shack...

"K9SQG" wrote in message
...
It might be simpler to use open wire feedline and a link coupled tuner or
tuner
with a balun in the shack...

The whole idea is that he does not need a tuner, except possibly the high
end of 10m, and the radio's built in tuner should suffice for that.

BTW, you do not want to use RG58 in lengths greater than about 2 meters. If
you want to keep costs down, buy generic RG8 Foam.


Tam/WB2TT