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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