Reg Edwards wrote:
The trouble with scaling (as from 2m down to 160m) is that antenna conductor
diameters are forgotten about which can cause appreciable errors.

I dunno, Reg. My 20m rotatable dipole diameter is about ten times
bigger than my 2m 1/4WL ground plane. Seems about right to me.
--
73, Cecil http://www.qsl.net/w5dxp



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The VERT1 vertical that comes with EZNEC is 1/4WL on 40m.
It will be 5/8WL on 17m. (7.2 * 2.5 = 18) EZNEC sez the
feedpoint impedance on 17m will be about 100-j470 ohms.

Hi Cecil, He was asking about 24MHZ which is 12 meters. EZNEC sez a 5/8 on
24MHZ is 63-j206, more or less. As others have pointed out a lot of factors
will enter into the final installed impedance.
Another matching method, for 63-j206, is feed the antenna with a 1.2 meter
length of 300 ohm twinlead- 1:1 balun- 50 ohm coax. The twinlead transformer
gives 43-j0. Don't tell Reg, but I did that on the Smith Chart in 48.32
seconds:-).
73 Gary N4AST

JGBOYLES wrote:
Hi Cecil, He was asking about 24MHZ which is 12 meters. EZNEC sez a 5/8 on
24MHZ is 63-j206, more or less.

I had forgotten I had changed VERT1 from tubing to #14 wire.
That was the problem with the higher impedance. With 3 inch
dia. vertical tubing the impedance on 18 MHz is 62-j190.
--
Sorry & 73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote in message ...
Larry D wrote:
Can anyone tell me the impedance I can expect to see,
also any tips on matching it? Thanks!

The VERT1 vertical that comes with EZNEC is 1/4WL on 40m.
It will be 5/8WL on 17m. (7.2 * 2.5 = 18) EZNEC sez the
feedpoint impedance on 17m will be about 100-j470 ohms.

This is what I ran when I had my 40m GP up. It worked both 17m and
40m.
My old elnec model of that antenna showed appx 48.4-j213 ohms on 17m.
But I had sloping radials. Maybe that made a difference. I used an
ungrounded series coil to tune for 17m, and the SWR was very low. I
had a 24v DPDT relay that I used to bypass the coil for 40m use.
Otherwise, it would be resonant about 6.9 mhz. I had the transformer
here in the shack, and when I changed bands, I would just plug, or
unplug the transformer from my power strip. MK

On 14 May 2004 09:45:16 -0700, (Mark Keith) wrote:

My old elnec model of that antenna showed appx 48.4-j213 ohms on 17m.
But I had sloping radials. Maybe that made a difference.

Hi Mark,

Yes, the significant difference. I have always endeavored to build
5/8'ths with an offset dipole fashion either with sloping radials, or
elevated feedpoint on a metal mast that terminated in a ground field
(radials). As this is also the basic structure of the sleeve dipole
(grounded at the bottom), using coax with a ground tap to the shield
about quarterwave (a guesstimate) from the feed made it easy to
achieve.

73's
Richard Clark, KB7QHC

Z is close to +58-j138 ohms.

A small inductor of 0.88 uH in series with the antenna should provide a
pretty good match to 50 ohm coax.

Larry D wrote:

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

Dave Shrader wrote:
Z is close to +58-j138 ohms.

A small inductor of 0.88 uH in series with the antenna should provide a
pretty good match to 50 ohm coax.

Larry D wrote:

I want to construct a 5/8 wave ground plane for 24 MHz.

Wow, after all that, a simple answer. Thanks!

The reason I wanted to try 5/8 wave is because I have had
the Solarcon A99 1/2-wave ground plane for 10 meters up
for about 5 years, and it is an excellent antenna for DX. This
is, according to the product blurb, a "1/2 wave, voltage-fed"
design.

The radiation patterns in the antenna book give the
impression that 5/8 wave should be "even better" for DX.

Of course, a lot of that performance had to do with who is
on 10 meters in the first place, and conditions over the past
5 years as well.

It is finally falling apart (radials coming loose,) and the
prospects for 10m in the next few years don't look so good.
I wanted to switch to a similar antenna, but for 24 MHz.

I notice that some of the answers here were geared toward a
ground-mount, which is not what I meant by "ground plane,"
this antenna will be at 30', and probably will have full-size,
1/4 wave (?) sloping radials.

As it turns out, at the antenna forum in Dayton, Dean Straw,
the author of the ARRL antenna book, talked me out of using
5/8 wave. He feels that it will not appreciably enhance DX,
compared to 1/4 wave.

He also talked me into getting the latest version of the
antenna book, which alas, still does not mention doing what
you suggest, not even in the VHF section, as one person
mentioned, but at least I have the answer now. Thanks again.

Larry DiGioia N8KU

Larry D. wrote:
"The reason I wanted to try 5/8 wave is because I have had the Solarcon
A 99 1/2 wave ground plane for 10 meters up for about 5 years and it is
an excellent antenna for DX."

OK. A comparison of unattenuated field strengths at one mile from
various verticals is given on page 20 of the "Vertical Antenna Handbook"
by Capt. Paul H. Lee, USNR, K6TS:

1/4-wave, 196 mv/m

1/2-wave, 236 mv/m

5/8-wave, 276 mv/m

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.

A 5/8-wave at 10 meters is 1/2-wave at 12.5-meters (24 MHz).

A 1/2-wave at 10-meters is 0.4 wavelength at 12.5-meters (24 MHz). One
could resonate it with a series capacitance at the antenna.

Best regards, Richard Harrison, KB5WZI

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

73
Danny, K6MHE

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.


Dunno. My real world tests don't quite agree. In using 30 mile ground
wave tests across town, I tested 1/4 GP's, 1/2 waves including
decoupling sections, and a 5/8 GP with 3/4 wave radials. All at 36 ft.
The 5/8 ate the 1/4 GP for lunch. Probably 2 plus S units better than
the 1/4 GP. The 5/8 beat the 1/2 wave by 1.5 S units. And this was
tested and repeated over a period of months. Never varied. Ground wave
testing is quite stable, and accurate for those low angles involved.
Much more accurate than trying to compare using constantly varying
skywaves. In real world gains, thats more than 1 db. 5/8 antennas are
weird animals. On 2m, they suck. On HF, they can do fairly well, cuz
the angles involved are not as critical. I used a 5/8 GP on 17m for
2-3 years. "also at 36 ft at the base".
It mangled every other antenna I had on that band. On 10m, the 5/8
beat any other length radiator quite handily. Again, on the critical
2m band, peeeyooooo.....they stink. BTW, on skywave, using a quick A/B
test, all preferred the 5/8, over the other antennas. So it wasn't a
low angle ground wave fluke. MK