RadioBanter - View Single Post

[email protected] · October 31st 14, 06:52 PM posted to rec.radio.amateur.antenna

Frank Turner-Smith G3VKI wrote:
On 31/10/14 01:17, wrote:
Ralph Mowery wrote:

"Ian Jackson" wrote in message
...
The original proposal in this thread was that long antennas performed
better than short ones. If that was true you'd get a good 600MHz UHF TV
picture using a 132ft end fed longwire. I've not tried it, but it doesn't
seem very likely.

A 132' endfed will have one hell of a gain on 600MHz - but it will be
almost straight off the ends.
--
Ian
I know the gain will be off the end of the wire, but still wonder if an
antenna that long (in wavelengths) will actually work or will it be too
long and the gain does not meet the expectations or if programs like NEC
will predict it or fall apart.

EZNEC handles it just fine.

I modeled a 120' (I had forgotten the exact number posted) long wire
at 6' over real ground at 600 Mhz:

Impedance: 55-j308
Max vertical gain: 21 dBi at 4 degrees
Horizontal gain: two 21 dBi lobes at +/- 4 degrees
Front/back: 9.5 dB

LOTS of little lobes...

Interesting, and I assume the -j308 is due to the capacitance between
the wire and ground. Since we are discussing a 50cm wavelength I would
imagine a change of only a cm or so in the overall length of the wire
would cause a significant change in impedance.
Another problem could be finding a big enough plot of land facing in the
right direction.

I did a little playinng around...

Changing the length from 120' to 132' has little effect on the pattern
as this is on the order of 80 wavelenths.

As the wavelength is so short, the impedance varies greatly with small
changes in length and the reactive part is heavily influenced by the
height above ground and the quality of the ground.

So in addition to finding enough land, you would also have to keep it
always wet or always dry otherwise you would be constantly retuning
as the ground moisture changed.

However, if one lived right on a beach and set up a series of floats
across the water...

Then your only problem is waves causing changes in height.

--
Jim Pennino