[TowerTalk] Vertical dipole other choices?

[jimlux]

On 10/20/20 1:50 PM, David Gilbert wrote:
> 
> 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.

Well, that's a matter of matching networks. And if one presumes an 
adjustable matching network at the base (or near the feed), then the 
concerns about Rrad and narrow band go away, for a reasonable size (i.e. 
no 1 meter dipoles on 160 m)

Rrad of a short dipole is 20 pi^2 * (L/lambda)^2

So looking at the case of a 8 meter long (26 foot) dipole on 80 meters, 
L/lambda is 1/8, the Rrad is about 2 ohms..

A 26 foot aluminum tube 2" in diameter has an AC resistance of about 
0.02 ohms.. So you're still in the "few percent loss" bucket, as long as 
your matching network can handle the transformation.

Which you've looked at in the following paragraph


> 
> 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. 

yes.. one might want to look at other matching techniques (like maybe a 
transformer) -but that just fixes the R component. You're going to have 
to deal with the 3000j. that's about 300 microhenry, which isn't too 
big. 4" in radius, 4" long, with 40 turns (10 turns/inch) - that's close 
to 1.1 ohms AC resistance.. 30% loss?

Going to even something impractical (1/2" diameter conductors) doesn't 
save you much (resistance is about 1/5th)



  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.
>>
> 
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