Dan Richardson wrote in message . ..
On Mon, 17 May 2004 10:42:49 -0500, (Richard
Harrison) wrote:
[snip]
Power varies as the square of the voltage. One can see the difference in
the field strength is hardly worth the effort for an amateur to try to
increase the length of his antenna. It`s about a 3 dB gain from 1/4-wave
to 5/8 wave.
[snip]

The 3 dB gain figure is valid when mounted on theoretical perfect
ground. For a ground-plane elevated above real ground you'll find the
gain to be rarely greater than 1 dB.

Modeling 1/4 & 5/8-wave ground planes at 30 feet above ground (@ 24
MHz) as was described earlier NEC reports the following maximum gain:

1/4-wave 2.35 dB

5/8-wave 3.06 dB

Then howcum my 5/8 wave mag mount 2m mobile antenna very significantly
outperforms the 1/4 wave mag mount antenna I used to use? Same ground
condx, same power, same feeder length, same vehicle, etc. I suspect
it's in the differences in the TO angles.

73
Danny, K6MHE

Brian w3rv

Larry, N8KU wrote:
"I want to construct a 5/8 wave ground plane for 24 MHz.----Even the
ARRL antenna book hasn`t been much help."

At 1/2-wave, reactance nearly is zero and begins repeating as from
zero.. With a 5/8-wave, you have a 1/8-wave connected to the feedpoint
through a reactance repeating 1/2-wave line. A 5/8-wave antenna will
have nearly the same reactance as a 1/8-wave. Value of the reactance
depends on the characteristic impedance of the antenna and this depends
on length to diameter ratio.

Resistance of a vertical antenna increases with its length until it
reaches nearly 1/2-wavelength where it peaks and starts to fall.
Sharpness of this resonant peak depends on length to diameter ratio of
the antenna. You might expect a radiation resistance of between 50 and
150 ohms. Reactance may be between 200 and 1200 ohms. It depends on how
skinny the antenna is. Fat conductors have smaller impedance variations
(a lower Q).

My impedance numbers come from Capt. Paul H. Lee, USNR, K6TS`s "Vertical
Antenna Handbook".

5/8-wave vertical ground-planes were once popular on 27 MHz and often
used an autotransformer at the base to step up the impedance from the 50
ohms of the coax.

Best regards, Richard Harrison, KB5WZI

Several posters have recommended a simple series coil for matching.
But of course, you'll never get lower than the resistive component of
the feedpoint impedance that way. At least one suggested a tapped
coil, and that can give you a good match and has the advantage of
DC-grounding the lightening ro...I mean radiating element.

Another alternative is a shunt coil to ground and a series capacitor
off to the feedline; that gives you the grounding and also lowers the
low-frequency coupling to the feedline. If Reg's suggested 110-j386
is correct for your case, you can get a match to 50 ohms at 24MHz with
a 1.1 microhenry shunt coil and 25pF series capacitor. The Q should
be around 5, not much higher than the antenna itself but can lead to
fairly high voltage across the cap. Don't leave it exposed to rain
:-) (That's an advantage of the tapped coil: less suseptable to
weather problems.)

But beware...the range of impedances I've seen posted suggests your
installation may be somewhat different.


Cheers,
Tom


Larry D wrote in message ...
I want to construct a 5/8 wave ground plane for 24 MHz.
I am OK with the dimensions, but I can't get a grip on what
the impedance at the feedpoint will be. Even the ARRL
antenna book hasn't been much help.

Can anyone tell me the impedance I can expect to see,
also any tips on matching it? Thanks!

Larry DiGioia N8KU
N8KU at longwire.com

"Larry D" wrote in message
...
I want to construct a 5/8 wave ground plane for 24 MHz.
I am OK with the dimensions, but I can't get a grip on what
the impedance at the feedpoint will be. Even the ARRL
antenna book hasn't been much help.

Can anyone tell me the impedance I can expect to see,
also any tips on matching it? Thanks!

Larry DiGioia N8KU
N8KU at longwire.com

Larry, its been a while since I built one but I usually wind an 1/8 to
3/16wl of wire on my matching coil and feed this in series with as one would
a base loaded antenna then start triming the coil for lowest SWR. I have one
I built like that now on 10M SWR is about 1.5:1 . Cant say for sure but
think the feed point impedance is above 50 or about 75 ohms at resonance.
Have built these using the same technique for 6 and 2 . Seems like the
impedance on these can vary quite a bit depending on the materials you build
the antenna from. Thicker materials seems to give a lower impedance and a
better match to 50 ohms. Ive never really gone into what was going on with
the antenna that much and may be all wrong but right or wrong my way of
doing it seems to get the job done for me. Perhaps some of the gurus around
here would like to provide some more insight.