From: andrew@rentec.com (Andrew Mullhaupt)
Newsgroups: rec.radio.shortwave
Subject: Re: Antennas: general question
Date: 12 Jan 93 21:02:58 GMT
Organization: Renaissance Technologies Corp., Setauket, NY.

In article <CB.93Jan11165530@tamarack13.timbuk> cb@tamarack13.timbuk (Chris Brewster) writes:
>
>To sum up my question, what's the best antenna to get?  I hear lots of
>discussion of antenna types, but significant voices (such as Harry
>Helms) keep saying "none of those things will do any more than a
>long-wire".

Well he lives on some huge flat estate or else he's lying. If you can
afford to put a big wire way up in the air for a long distance, then
that's just ducky. Way up means as much as 100' high and big means
hundreds of feet. Then rhombics, Beveridges, and long wires are just
great.

In the case where you live in the real world, it is quite likely that
you will do better with a tuned sloper or dipole than with a random
wire given equal amounts of installation difficulty. You cannot effectively
design and build your own electrically tuned antenna for less than you can
buy one such as the Alpha-Delta or Eavesdropper. However, there are many
things that deserve thought.

1. Can you put up a moderately large outdoor antenna? If so then you
probably should investigate the commercially available antennae. Safety
is very important here. Make sure you get good information before exposing
yourself to risks of falls, fire, lightning, or even damage to your radio.

2. Do you live in an area with a lot of electric/electronic noise? In
which case active antennae may not help much at all.

3. Do you have a big table radio or a little portable? Little portables
are designed to be connected to little vertical whip antennae, and may not
do well with their sensitive front ends connected to high gain antennae.
Even if you don't damage your radio, exposing it to strong signals may
overwhelm the circuits it uses to separate the desired signal from unwanted
signals. Since this sounds to the listener like listening to all radio
stations at once, it probably is to be avoided.

4. If you live in an electrically quiet area, an active whip may be much
more convenient and in some senses safer than a big outdoor antenna.

5. Grounding can be important but hard to understand. It can even depend
on your local soil conditions, etc. Sorry.

>[I've been away from shortwave for a long time.  I bought a used Icom
>R-71A, but I'm not getting the results I think I should get--only the
>really powerful mainstream services, whereas the things I want to hear
>are at least a little hard to get, such as Iraq.  There's some chance
>that the Icom isn't up to spec, but it *looks* immaculate.]

OK you can go hog wild on antennae since your set can handle it and
should give better results than you are reporting.

>I'm using the Eavesdropper antenna, which uses traps to resonate at the
>broadcast bands.  I've heard that traps and coax lead-ins both entail
>some loss, so I'm going to change, but to what?

I wouldn't worry that the traps are the problem - is your antenna installed
properly, is the coax in good shape? One of the problems with Eavesdroppers
that has ocasionally been reported is loss of integrity of the coax due to
weathering, etc. This can eat up a big chunk of signal. Anything which
electrically tunes the antenna to resonate can be thought of as causing
'loss' but the point is that it is supposed to lose more unwanted signal
than wanted signal. What you should be concerned about is the signal to
noise ratio, which is _improved_ by proper resonance of the antenna with
the desired signal. Almost certainly your results would get _worse_ if you
just cut out those traps. Ditto for the coax - it causes loss, but it prevents
local signal from getting into the feed line. It's doing fine if it keeps
bad stuff out more than it loses good stuff, etc. Don't think that overall
signal level is the most important aspect of the antenna system - it isn't
at all with today's sensitive radios. A quick glance at the Universal
catalog shows that the R-71A spec is _quite_ sensitive.

Ask yourself this question - what is the ambient noise floor where you listen?
Is your radio sensitive enough to pick up the spurious signals the house
next door, etc. are generating if you tune to an 'empty' frequency? If so,
then boosting the signal level by amplification will not improve the signal
to noise ratio - and if not done very well it will degrade it. You should
be careful to identify what is the thing most responsible for your poor
results. It may be low signal level, but this would almost certainly
indicate that either your radio or antenna was disfunctional. If both are
in good shape, it's quite possible (even likely in urban areas) that you
are getting down to the noise floor, and the only help for better listening
is to purify the signal your radio sees by discriminating against noise.

Keep in mind that the radio functions as part of a system which includes
the transmitter, your radio and antenna, _and_ the intervening atmosphere.
The signal to noise ratio of the atmosphere varies _drastically_ with
time of day, frequency, season, and solar activity. Not a lot you are
likely to do about these factors, either... So there are some clear limits
to HF radio communications which should be thought through.

>Page 44 of Passport advertises Alpha Delta's DX-SWL antenna:  "Efficient
>multiband frequency selection by means of special RF choke-resonators".
>Is this any better than a tuned long-wire?  How important is tuning?

In your picture, tuning may be _very_ important, but pointing the antenna
in the right direction, its polarity, etc. are probably just as important.

>One more question: I wonder if it would make sense, even with a simple
>antenna, to use a coax lead, and mount a preamp outside at the beginning
>of the lead-in.  I was thinking this would be a way to prevent noise
>originating in one's house.

It can be, but it is also a way to inject amplifier noise into the signal
_and_ it may cause loss of sensitivity if the front end of your amplifier
is not very sensitive and the front end of your radio _is_. Since you have
an R-71 you should have ample sensitivity to get down to the noise floor.

Ever since they invented highly sensitive front ends for radios (e.g. using
FETs) it hasn't been too important to amplify the antenna unless you do
a _quality_ job of it.

As far as passive antennae go, the more metal you can get up in the air
without detuning the antenna, and all other things being equal, the better.

Note: I'm not satisfied with the antenna technology of today. Comparing
the advances in radios with those in antennae show that HF antennae are
not being thought through well enough. If nothing else, it would be nice
to have electrically steerable/polarizable antennae controlled by a computer
with feedback from the radio, and this is pretty do-able with present
technology. People often spend hundreds of dollars on radios, but there
doesn't seem to be a corresponding possibility for useful investment in the
antenna. I think this important subject should get more attention in the
SW community.


Later,
Andrew Mullhaupt
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