(Steve) wrote:

When people talk about the 'clearance' of an antenna, they seem to
mean how high the antenna is suspended above the earth and surrounding
objects. It also seems to be agreed that, at least with most types of
antennas, the more clearance you have, the better.

With many, yes. However there are certain antennas where the ground
(earth) plays a role in the reception.


If I were going to put a random wire antenna on the roof of my five
story building, would the elevation resulting from its rooftop
location be a plus even it's only six or seven feet above the rooftop
itself (assuming there's not a lot of rfi up there from power lines,
the apartments below, etc.)?

Good question. Don't know. It probably wouldn't make that big a
difference. People have used antennae laying on the ground with good
results.

With a random wire, the ground system is important, so that may be a
deciding factor. Can you lay a counterpoise or two on the roof?


Second question: I'm thinking the random wire should attach to a balun
which should then attach to a coax feedline. (My understanding is
that, if the wire isn't cut for a particular band, I'll need to use
either a balun or a tuner-- and it would be nice to avoid having to
fiddle with a tuner all the time.)

Okay, pedantic mode he What you'll be using is an un-un or a
matching transformer, not a "balun," despite the marketing hype
surrounding one particular company that loves the term.

For an end-fed wire, you actually DON'T want it to be resonant.

You also don't need a tuner -- see below.

But how does one determine what
sort of balun to use? For example, how does one decide whether to use
a 4:1 vs a 9:1 balun?

Experiment and try. I'd recommend the I.C.E. matching transformer
with multiple taps on it plus a ground lug -- the latter being crucial
for minimizing noise pickup.

The deal is that the impedence of the antenna at various non-resonant
frequencies is gonna be all over the place (with real and imaginary
components). What the transformer will do is give you a good
compromise choice that will minimize signal loss across your bands of
interest -- remember, your antenna is not resonant, nor should it be.

Can anyone recommend a particular balun that
they've found to work well with a random wire antenna?

I can't recommend I.C.E. products strongly enough. They are high
quality, have the right features, and do the job well. They don't try
to bull**** you with marketing either.

I.C.E. = Industrial Communication Engineers.

http://personal.isla.net/ice/
1-800-ICE-COMM

You'll probably have to call since their online catalog seems to be
incomplete. You are looking for a receive-only end-fed wire matching
transformer for HF.


Hope this helps


Thanks,

Steve

--
Eric F. Richards,
"Making me root for a sanctimonious statist blowhard like Kerry isn't
the worst thing Bush has done to the country. But it's the offense
that I take most personally."
-- http://www.reason.com/links/links071304.shtml