Hi Peter,
At 12:34 PM 12/8/04, Peter Koehler wrote:
Bill Tippett wrote:
I wouldn't be surprised if it was mostly high
angle beginning at Milt's sunrise.
Yes- very true! They were coming in better on my 80m Half-Square than the 160m
Verts or the beverage.
I have to ask this; In leu of the VK9 and the HS operations, what can be
better done at the receive end to help these folks copy better (other than
bigger amps at our end). Many morning I've heard powerful stations call but go
unanswered. I can lament with Larry that only the most powerful stations have
made the contacts so far. I hope the propagation Gods will smile upon us
9V1GO and I have been having a private discussion about this.
For reasons I don't fully understand, it appears that using very low
dipoles for RX at the SE Asia side of these paths seems to work best.
Robin WA6CDR first discovered this at XZ1N and XZ0A. Bob 9V1GO
has confirmed it since he has been in Singapore (both at his sunrise
and sunset), and now we see it confirmed again by HS72B and VK9XG
both primarily using low dipoles for RX. In fact, on 9V1GO's suggestion,
VK9XG put up a 6' dipole which became his primary RX antenna when his
Flag or ??? that Charlie was using before did not work.
The only S/N directivity a low dipole has is to extremely high
angles...i.e. there is no azimuthal directivity. Perhaps most of the
local noise is low angle and the DX signals are high angle, which gives
the low dipole good S/N for high angles. The only other issue I can
think of is polarization which might be an effect at Equatorial locations
like 9V, HS, XZ, VK9X, etc (i.e. electron gyrofrequency effect), but I
personally feel the issue is mainly one of takeoff angles.
The unanswered question is, is this reciprocal for both TX and
RX, or is it RX only? The problem with a very low dipole is that it is
also very lossy due to ground absorption. This does not hurt RX but
it DOES degrade the TX signal by ~8 dB To make a good high angle
TX antenna, a dipole should be at about 50' and an inverted-V should
be at about 100' (according to Eznec models). Bob 9V1GO is going
to put up a horizontal loop, mostly over ground at about 100', in the
next few days and see if high angles work for TX as well as RX. The
bad news for expeditions is that it is actually MORE difficult to put up
a GOOD high angle radiator than low angle radiator (i.e. the difference
between a dipole at 50-60' versus a single vertical).
As far as advice to expeditions in exotic locations, here are
my thoughts:
1. Find as remote a location as possible (away from city noise, etc).
2. Find a location where good RX antennas like Beverages are possible.
3. Put up as large a vertical as possible with a good ground system.
4. Try a very low dipole for HIGH angles in addition to #2. (Beverages,
Pennant/Flag, EWE, K9AY all being good RX antennas for LOW angles.)
Many of us still remember 9M0C's great TX signal a few years
ago when they were stone deaf (one QSO with the East Coast). I often
wonder how they might have done if they had discovered the low dipole
trick that Robin used from XZ. Will be interesting to see how 9V1GO's
experiments turn out.
73, Bill W4ZV
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