On 26 Jan 2005 22:35:18 GMT, Allodoxaphobia
wrote:
On Wed, 26 Jan 2005 21:27:31 GMT, W9DMK wrote:
On Wed, 26 Jan 2005 15:05:39 -0600, "Ken Bessler"
wrote:
My 160/80m trap dipole can be operated without
a tuner from 1.800 to 1.930. I mainly operate in
that range (usually on 1.893) and I have a tuner for
operating above 1.930. Easy.
My question is this - is it better to resonate high and
use a tuner to go lower or do it the way I'm doing now
(resonate low and use a tuner to go higher)?
Or will it even matter enough to notice??? My guess
is (at these freqs) no......
Dear Ken,
Your trap dipole is resonant only in the sense that you see
approximately 50 + j 0 ohms at the input to the feedline. I will
assume that it is not actually of the length of a "resonant dipole"
(about 260 ft.).
Therefore, we are talking about a "resonant" antenna/feedline
"system".
The next issue is the radiation pattern of the antenna. Regardless of
how you use it (i.e., with or without a tuner), it will have a pattern
that is a function of its configuration and operating frequency.
Whether or not a tuner is involved is not a factor.
The only real issue is efficiency. Everything else being equal, the
tuner is going to rob you of some of your power. How much will depend
on many factors - the primary ones being the actual impedance seen by
the tuner and the specific design of the tuner - especially its
inductor.
Therefore, it would appear that you are doing the best thing - namely,
to use the tuner only when you have to.
However, I would like to see an answer to his question:
1. Is it best to cut "high" and tune for "low" when necc.?
-or-
2. Is it best to cut "low" and tune for "high" when necc.?
-- when operating on a band (i.e., 75/80M or 160M) where a 50 to
100 kcs. shift in freq. is a large'ish percentage change in
wavelength.
3. ..or, is it Mox Nix?
I don't consider 50 - 100 kHz to be a sufficiently large shift to make
any difference. It's only 2 1/2%, which is nothing.
So, you are correct - es macht Nichts aus!
Remember, "resonance" is not all it's cracked up to be. In fact, it's
almost irrelevant at the typical heights above ground encountered (45
ft or less for 80 and 160 m). I'm not saying that you do not need to
cut a dipole to the frequency. What I am saying is that at low heights
all the power goes straight into the clouds, anyway. The reason most
people like "resonance" is for the ease of matching to the rig's
output stage - not that the antenna works any better. You probably
don't care in which direction in the horizontal plane the energy goes
- it's going to be wrong half of the time, anyway - right?
Bob, W9DMK, Dahlgren, VA
Replace "nobody" with my callsign for e-mail
http://www.qsl.net/w9dmk
http://zaffora/f2o.org/W9DMK/W9dmk.html