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

I'm not sure about your 50 + j43 for a 0.26 wave. Is that from EZNEC? With
what kind of ground?

Yes it was EZNEC, ground type real/Mininec.
73 Gary N4AST

"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.

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

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

On 18 May 2004 06:19:50 -0700, (Brian Kelly) wrote:

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.


My previous response was for a ground plan antenna mounted above
ground and you are addressing a mobile installation. They are
different.

I've done modeling of 1/4,1/2 and 5/8-wave whips using several wire
grid models for vehicles (small and mid-sized car, small pickup truck
and a SUV). I found that the vehicle's size, shape and whip location
plays a major part in performance.

I also noted, on the average, the 5/8 produce a slight gain over all,
but it was possible to find azimuth directions that a 5/8-wave would
produced almost 3 db gain over itself - depending what vehicle it was
mounted on.

The most outstanding feature I saw was a 5/8-wave whip azimuth pattern
was less influenced by the vehicle geometry.

The operation is more like a lop-sided dipole with the vehicle body
being on leg.

I don't feel you can accurately predict how a the whips will perform
on a vehicle based upon operation on another vehicle - unless both
vehicles and the antenna locations are the same.

I should add for all models the whips where placed top-dead-center of
the vehicle's roof.


73,
Danny, K6MHE

I agree completely. Question is, if modeling and prediction is so unreliable
why do we bother?

"Dan Richardson @mendolink.com" ChangeThisToCallSign wrote in message
...
On 18 May 2004 06:19:50 -0700, (Brian Kelly) wrote:

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? ....

My previous response was for a ground plan antenna mounted above
ground and you are addressing a mobile installation. They are different.

... I found that the vehicle's size, shape and whip location
plays a major part in performance.

.... it was possible to find azimuth directions that a 5/8-wave would
produced almost 3 db gain over itself ....

I don't feel you can accurately predict how a the whips will perform
on a vehicle based upon operation on another vehicle....