[TowerTalk] Folded dipole

[Martin Ewing]

On Tue, Aug 25, 2009 at 9:54 AM, jimlux<jimlux at earthlink.net> wrote:
> Tom Osborne wrote:
>> Hi All
>>
>> Looking to put up another 40 meter antenna.  Need a NVIS antenna for
>> close-in work.
>>
>> Is there any reason to put up a folded dipole instead of a tuned-feeder
>> dipole?
>>
>> I think when this antenna was popular, it was touted to have more bandwidth
>> than a dipole.  But, if you have to use an antenna tuner to tune it anyway,
>> there doesn't seem to be any advantage over a regular antenna, bandwidth
>> wise.  73
>
>
> The folded dipole *might* get the impedance to a better value for the
> tuner.  To a first order, a folded dipole has 4x the feedpoint impedance
> of the straight dipole.
>
> If you were talking about the terminated folded dipole (e.g. the things
> from B&W, among others), then the loss in the antenna makes it a
> broadband unit without a tuner.
>
> A lot of practical systems use the lossy dipole approach so they don't
> have to use a tuner, and just make up for the loss with a bigger power
> amp.  If you have a wide band to tune over (e.g. supporting emergency
> comm with a NVIS antenna, and you might have to tune anywhere) this
> isn't a bad approach.
> _______________________________________________
>

I ran a B&W terminated dipole for years, and got some useful results.
(when there were sunspots) However, it has a gain of something like -6
dB, depending on frequency.  I.e., about 75% of your power goes into
the terminating resistor.   The good news is that we often have more
than 6 dB of signal margin on HF.

The idea of matching through resistance does help -- you don't have to
put up multiple antennas, but there is a pretty big cost in
performance.

73 Martin AA6E