[TowerTalk] Vertical dipole other choices?

[David Gilbert]

Very true, and that's essentially what N6BT's ZR antennas were.

In general, of course, the feedpoint impedance goes down for a shortened 
antenna, which typically means the feedpoint efficiency suffers, and the 
bandwidth gets a lot narrower.

If you can effectively get the power to a shortened antenna the radiated 
energy is not a lot different than for a full sized antenna, either for 
magnitude or pattern.  I just modeled a 25 foot vertical dipole for 80m 
with the bottom two feet off the ground. The main lobe is 0.77 dbi at 27 
degrees which is almost identical to a full half wavelength vertical 
dipole, but the feedpoint impedance is 3 - j3000.  That's going to be 
really hard to match without a ton of loss.  TLW's tuner calculator says 
that an L-Network with an inductor Q of 200 and a capacitor Q of 1000 
would have over 10 dB of loss and result in a 6 KHz bandwidth.  Even an 
inductor Q of 400 is going to dissipate 1,000 watts in the inductor with 
full legal power feeding it, so I suspect such an antenna would be 
limited to more like 100 watts input.

73,
Dave   AB7E





On 10/20/2020 1:21 PM, jimlux wrote:
> On 10/20/20 1:01 PM, David Gilbert wrote:
>>
>> Electrically, a half wave vertical dipole is not a lot different than 
>> a quarter wave vertical fed against radials or a counterpoise, but it 
>> requires a LOT more height to put it up.  I just modeled two 
>> different antennas in EZNEC+ as a comparison (both over medium ground):
>
>
> A lot of these antennas are an electrically short dipole. so they 
> don't require the height. From a gain standpoint, an infinitesimally 
> small dipole is 1.5 dBi and a full size dipole is 2.5 dBi (mostly from 
> the broader lobe for the short antenna).
>
>
> What might be interesting is modeling, say, a 25 foot dipole with the 
> center 13-14 feet off the ground.
>