Tom Ring wrote:
Art
Which brand of EME is this for? If for one of the new digital modes,
you don't need that much gain, as you may already know. If it's for
CW, having a ton of gain in one direction only gives you a very seldom
available and very short window.
You would be better off sticking up about 15dBd of rotatable gain and
making a horizon sched with W5UN. The 2m EME net is on 14.345 at 11AM
central saturdays and sundays. Net control is VE7BQH. Dave, W5UN, is
almost always on.
Dave also does the digi EME modes, and as a guess could probably work
you on something like 10dBd with 100W on one of those.
I know Ian is an EME're, so pipe in, Ian!
(Still sleepy and jet-lagged...)
By coincidence, I was involved in just that kind of thing in 1979(?)
when we made the very first 2m EME QSOs from G-land using amateur
antennas at both ends of the path. One of our group was a farmer's son,
so we were able to string a 600ft rhombic over a large field of pigs.
The quick answer about feeding a very long V-beam on 2m is to use a
"universal stub" - a half-wave open-wire stub with a shorting bar, and a
4:1 coax balun. Adjust the tapping points for the shorting bar and balun
to get a good impedance match, and away you go. The universal stub is
almost a lost art, but any *old* VHF handbook will show you how to make
one.
It's obviously much more convenient if you come down to ground level in
high-grade open-wire feeder - not the store-bought stuff, but home-made,
with close-spaced wires under tension and a minimum of insulators. You
can then do the matching at ground level.
30 wavelengths per leg should be long enough to eliminate any
termination requirements at the far end. Radiation "loss" from the
forward-travelling wave will automatically ensure that the rear lobe is
reduced. If you wish, you can terminate the far end of each leg with a
300R low-inductive resistor and two quarter-wave "radials" in a T
configuration... but you'll probably not notice the difference.
However, it's true that:
having a ton of gain in one direction only gives you a very seldom
available and very short window.
This is a major inconvenience - you get maybe 20 minutes total operating
time per day, on maybe 3-4 days per month maximum. And that's only if
the direction of the beam is perfectly optimized. You need to lay out
the antenna with an accuracy of about 1 degree maximum, so you'll need
to borrow some serious surveying equipment. Guess-and-compass methods
will not work, because even small azimuth errors could mean that you're
operating on completely the wrong DAY!
(As the one who did the calculations, I can still remember the feeling
of relief on verifying that the moon really did set in front of the
rhombic, and on the right day too.)
Also, these moonrise or moonset windows will occur at arbitrary times of
day or night. With absolutely no time to waste, you will be limited to
making skeds... and not many sked partners may want to share that
inconvenience with you.
Overall, I agree with Tom - a large fixed antenna was the right thing to
do 20+ years ago, but 2m EME is now in a very different place.
A smaller steerable beam will trade raw gain for a huge increase in EME
operating *time*, and with modern operating techniques, time is what you
need the most.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek