5/8 wave ground plane
 

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

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.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

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.
----
.................................................. .........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. .........

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.

A loading coil is usually installed between the base of
the antenna and the radial system. The coax braid is
tied to the radial system and the coax center wire is
tapped down on the coil at the 50 ohm point.

Rad| coil
Gnd-------+-////////---------------5/8WL----------------
Rad| ^
|
XMTR-----------+

If you don't mind a relatively low SWR of 2:1-3:1, use good
coax, a standard base-loading coil, and your tuner at the
transmitter.

There is plenty of 5/8WL information for 2m antennas
in the ARRL publications and antennas are scalable.
--
73, Cecil http://www.qsl.net/w5dxp




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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

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?

==============================

The program takes into account antenna diameter. Does Kraus? And is he on
the same band? Is he at the same height above ground? What sort of ground
or radial system does he have? Have you interpolated correctly? Have you
understood what Kraus is saying? Is the loading coil in the same location?
Is the feedpoint at the same place. Are you comparing like with like?
Spend more time thinking about it.

Is Kraus infallible? Could be he's wrong again.

Who's Kraus anyway? ;o)
----
Reg

There is plenty of 5/8WL information for 2m antennas
in the ARRL publications and antennas are scalable.
--
73, Cecil

===========================

The trouble with scaling (as from 2m down to 160m) is that antenna conductor
diameters are forgotten about which can cause appreciable errors.
---
Reg, G4FGQ

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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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

"Reg Edwards" wrote in message
...
The program takes into account antenna diameter. Does Kraus?

Yes. My post said length/diameter = 472 (One thing I don't like about the
way Kraus wrote his book is that he gives the formula for only one part of
the feed impedance (R or X) and a diagram for the complex impedance. He
then refers you to someones paper somewhere for the 'complete' formula.. I
have it around here but it would take 2 days to find it.)


And is he on the same band?

In the diagram I was interpolating from the dimensions are in wavelengths.

Is he at the same height above ground? What sort of ground or radial
system does he have?

It is an infinite perfect ground plane---ground mounted. (We are talking
about a 5/8 wave vertical remember).

Have you interpolated correctly?

Probably not.


Have you understood what Kraus is saying?

Yes.

Is the loading coil in the same location?

No 'loading coil' in the Kraus example. He is discussing the feed point
impedance. A loading coil would be either across or in series with the feed
point.

Is the feedpoint at the same place.

Since the original post concerned a 5/8 wave vertical, I assumed it is at
the bottom, which is what Kraus is discussing.

Are you comparing like with like?

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. 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!

Spend more time thinking about it.

I have, which is why I looked it up.


Is Kraus infallible? Could be he's wrong again.


=8o Oh no!!! Kraus wrong!?

Who's Kraus anyway? ;o)

Oh, I don't know. Just some guy that signed my Master's Thesis 16 years
ago.


----
Reg



--
Jim
N8EE

to email directly, send to my call sign at arrl dot net