Partly because you used a conductor diameter about 16.5mm and Reg's
was 10mm. (Don't think I'd want to make a self-supporting element
that long and that thin, myself. Maybe 50mm at the base and tapering
to 25 or so?) That probably does not account for the whole
difference. Did you both use the same sort of "ground plane"? Height
above earth has only a slight effect.

Cheers,
Tom


"JLB" wrote in message ...
Kraus (2nd edition, p. 375) gives a different answer.

Interpolating from the diagram on p. 375 (assuming length/diameter = 472) it
is about 90-j250.

Why the difference?

--
Jim
N8EE

to email directly, send to my call sign at arrl dot net
"Reg Edwards" wrote in message
...
For a 5/8-wave vertical above ground at 24 MHz, conductor diameter 10
mm. -

Base feedpoint resistance = 110 ohms
Base feedpoint reactance = -j386 ohms
Base loading coil inductance = 2.6 uH
Coil diameter = 25 mm
Coil length = 33 mm
Number of spaced turns = 13
Wire = 14 awg = 1.64 mm diameter.

For other heights, frequencies and coil dimensions download in a few
seconds
program BOTLOAD for immediate answers.
----
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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

JLB wrote:
Hence my question. Are we? I recall that a 5/8 wave vertical was
'close' to
50 ohms at the feed point and could be 'adjusted' with an inductance
across
the feed point (with the feedline at a tap perhaps?).

Tuned to an electrical 3/4 wave...Basically, just tuning out the
reactance..

That is the whole
point is using such a 'magic number' as 5/8 wavelength.

Huh? The point of the 5/8 radiator is pattern and gain related, not feed
impedance, or matching.

If you look at the
impedance curves, though, you find that they are quite steep at this
point,
so a small error in length causes a large error in impedance, and some
tuning will be needed. The 5/8 wave is not a 'build it and go' antenna!

It's pretty close to it though. All you have to do is start off with a
"best guess" coil and test. If it's resonant low in freq, take turns
off. One at a time, if close. If it's high in freq, you need more
turns. Not really any more complicated than trimming a dipole for
resonance. After building a lot of those, I can guess the number of
turns needed in my head and come pretty close. Reg's guess of 13 turns
is pretty close if the form is 1-1.5 inch wide. Using my built in
guessing machine, I would have quoted appx 15-16-17 turns off the top of
my head , if the coil is made of #12 solid wire, and wound on a 7/8 inch
pipe former, "I use a piece of 7/8 inch copper tube as the former" and
then taken off the form. "On those type of coils, I make them self
supporting. The #12 wire is stiff enough to hold it's shape. Just
connect and clamp each end of the coil to hold it in place. MK

--
http://web.wt.net/~nm5k

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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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

On 12-May-2004, "JLB" wrote:

Hence my question. Are we? I recall that a 5/8 wave vertical was 'close' to
50 ohms at the feed point and could be 'adjusted' with an inductance across
the feed point (with the feedline at a tap perhaps?). That is the whole
point is using such a 'magic number' as 5/8 wavelength.

The resistive part of the Z of a 5/8 wave will closer to 50 ohms than it will be for a 1/2 wave, but
in most cases will be higher than 50. You may be thinking of a vertical close to 3/8 wave which
will be 50 -jx. The -jx (capacitance) can be resonated with a +jx in series for a 50 ohm feedpoint.

Ken Fowler

You may be thinking of a vertical close to 3/8 wave which
will be 50 -jx. The -jx (capacitance) can be resonated with a +jx in series
for a 50 ohm feedpoint.

Ken, we may be talking about two different things, but a 3/8 wave vertical
has inductive reactance, and the R component is no where near 50 ohms. A 2/8,
or 1/4 wave vertical has no reactance, shorter has -jx, and longer, as in 3/8
wave has +jx.
A .26 lambda vertical has about 50+j43 impedance, and the +j43 can be
cancelled with series 154pf at 24 MHZ. This is a common method of matching a
1/4 wave vertical to 50 ohms. Make it a little longer to look inductive,
50+jx, and cancel with a series cap.
According to all the theory I know on this stuff a 3/8 wave vertical will not
be as you describe.

73 Gary N4AST

(JGBOYLES) wrote in message
A .26 lambda vertical has about 50+j43 impedance, and the +j43 can be
cancelled with series 154pf at 24 MHZ. This is a common method of matching a
1/4 wave vertical to 50 ohms. Make it a little longer to look inductive,
50+jx, and cancel with a series cap.
According to all the theory I know on this stuff a 3/8 wave vertical will not
be as you describe.

It shouldn't be too far off. The usual method of tuning a 3/8 vertical
is also by using a series cap to tune out the reactance. Quite common
on 160m, etc where people run the "extended" inv L's that are 185-190
or so ft long. I've used them here, and with a series cap, the SWR was
quite low. The 3/8 impedance is higher than a 1/4 wave, but not enough
to really hurt things, or to make a l/c matching device needed. MK

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 13-May-2004, (JGBOYLES) wrote:

You may be thinking of a vertical close to 3/8 wave which
will be 50 -jx. The -jx (capacitance) can be resonated with a +jx in series
for a 50 ohm feedpoint.

Ken, we may be talking about two different things, but a 3/8 wave vertical
has inductive reactance, and the R component is no where near 50 ohms. A 2/8,
or 1/4 wave vertical has no reactance, shorter has -jx, and longer, as in 3/8
wave has +jx.
A .26 lambda vertical has about 50+j43 impedance, and the +j43 can be
cancelled with series 154pf at 24 MHZ. This is a common method of matching a
1/4 wave vertical to 50 ohms. Make it a little longer to look inductive,
50+jx, and cancel with a series cap.
According to all the theory I know on this stuff a 3/8 wave vertical will not
be as you describe.

Sorry. I mispoke on that. The 3/8 wave will indeed be inductive. However its resistance will be
closer to 50 ohms than it would be for a 1/4 wave. A 1/4 wave would have a resistance of 37 ohms.
I'm not sure about your 50 + j43 for a 0.26 wave. Is that from EZNEC? With what kind of ground?

I didn't take the time to model the 3/8 wave. Sorry.

Ken