Aloha to all,
Now have up the 160, 80, 20, 15 and 10 meter monoband
Gladiator ground plane verticals. Ground plane as all have
elevated bases -- on Radio Shack push up poles -- and also
have elevated radials, sloping down and away from the base
of the vertical element. On the three higher bands, the
radials actually slope away at about a 45 degree angle,
possible because of the height gained via the push up
mast. Both 160 and 80 units cover a LOT of territory with
full size quarter wave long radials; was able to get them up
enough that one can walk around under them near the antenna
base.
Have the 40 and 30 meter Gladiators in hand out here, and
will be getting them up in the next days, as the strong,
gusty trades have moderated a bit allowing for outdoor
effort again. Got both the 15 and 20 up in the last few
days between rain squalls and gusts above 30 mph. Would
probably not have been possible with the longer 30 and
40 antennas.
One of the more fascinating uses of these verticals is as
separate receiving antennas input to the BPF-1 socket of the
FT-1000D transceiver. CW listening is enhanced via a stereo/
antenna diversity mode. You can actually hear the signal fade
from the beam and become enhanced from the vertical at
times; and all the time, the two receiver, different polarization
stereo effect is neat! Unfortunately, you cannot work CW split
mode this way, must return to the main antenna for both
main and sub-recvr's/ 2nd vfo xmitng for split operation.
Obviously signals on the tribander are usually stronger than as
received with the verticals, but not always, but usually all day long
while the sun is still up. With the beam looking toward the
mainland, signals from Japan, Australia, etc, are stronger
from the vertical, of course.
While tuning these verticals, I relearned an elementary principal
about transmission lines when connected to loads of an impedance
different from the line impedance! What you measure as the vswr
bandwidth, and even the apparent resonant frequency of the
antenna load depends upon where you are along the path to the rig;
at least I think that is what I have observed! See the following
discussion of some measurments I have made.
There are Top-Ten, Alpha-Delta and even one Hofi coax switch, a
low pass filter, an RF Applications power/vswr meter (P-3000),
and the XMatch tuner along the way before hitting the output
port of the 87A, through there to, at last the main antenna
socket on the 1000D. The remote Top-Ten switch selects which
vertical, then about 220 feet of LMR 400 line, selected to reduce
losses, an Alpha Delta which decides if the antenna goes to the
main rig line, or short cuts straight to the BPF-1 external rcvng
antenna socket, etc. There are others to deal with two different
tuners, and which of two transceivers are to be used!
I measure entirely different vswrs and apparent resonant frequency
areas depending upon where one measures in the
chain. But, in general, these verticals show a wide bandwidth of
low vswr. For example, on 10 meters I measure 1.0:1 vswr, on the
P-3000 between 28084 and 28384, or 300 kHz BW of flat bandwidth!
The swr is 1.1:1 between 28000 and 28540; this caught my attention
and caused me to go on a search for a 50 ohm load resistor somewhere!
All of these measurements have the XMatch set at DIRECT, so no added
reactances there.
I switched to the 20 meter vertical and did extensive measurements with the
MFJ meter in three different locations: one at the output end of the
220 feet of LMR 400 running to the shack from the Top-Ten switch
out in the antenna field; another directly at the coax connector which
would screw onto the main antenna socket on the back of the 1000D,
and then again on a single pole/four throw Alpha Delta that selects the
destination of signals which have passed through the XMatch antenna
tuner. There are a few feet of RG8 size coax in between the switches,
filter,
P-3000 coupler, linear and 1000D. I thought perhaps I would find some
obvious piece of bad coax or really lossy switch somewhere. Nothing
obvious.
Results: With the MFJ directly at the end of the LMR feed line,
I measure an apparent resonant frequency of about 14113,
with a vswr of about 1.3:1. At 14000, measure 1.45 or so,
and at 14240, again back up to about 1.4:1, and on to
1.5:1 at 14350, the top of the 20 meter band.
With the MFJ connected in place of the FT-1000D, that is to the
same piece of coax that connects to the transceiver, the
apparent resonant frequency, or actually the band across
which I read a 1.0:1 vswr, is about 14180 to 14235, a range of
about 55 kHz !? At 14000, it reads 1.5:1, and at 14350, 1.3:1.
If I set the MFJ meter aside, and instead fire up the 1000D, and
use the P-3000 to read vswr, again with the tuner bypassed,
I find the area of 1.0:1 bandwidth to be 14080 on up to 14182,
or a band twice as great and just about 100 kHz lower than
the MFJ reads at the end of the coax going in to the 1000D
main antenna input socket.
The 1.1:1 band measured this way is 14025 on up to 14240;
at 14000, 1.2:1 and at 14350, 1.4:1. Perhaps the placement of the
P-3000 coupler, where the vswr is being measured is the cause
of the difference? The directional coupler of the P-3000 is located
in the signal chain at the end of about 18" of RG8 type coax from the
RF power output socket of the Alpha 87A; from the P-3000 coupler
box there is another couple feet of coax which connects to the
Bencher Low Pass filter, after the filter the coax goes off to some
switches, etc. So the place where the measurements are being
made are different. P-3000 pick off coupler is away from the
1000D antenna port by the length of line between the 1000D
and the 87A, and then the 18" or so from the 87A to the coupler.
Have no idea what the length of signal path is inside the 87A when
it is off, and the path simply passes through.
What is read on the meter of the 1000D? With the LMR 400
connected directly to the back of the D, the vswr, as read
on the internal Yaesu meter is 1.3:1. All in the shack switches,
filters, couplers, etc are bypassed. The vswr falls gradually to
1.2:1 at about 14090, and remains at that value all the way up
to 14349! If I do the same measurment, using the D meter, with
the complete in-the-shack switch, filter, coupler, linear lash up,
the measurements are nearly the same: 1.2 at 14001, decreasing
to 1.1 at about 14050, and staying at that value to about
14115 where the vswr on the Yaesu meter now reads 1.0, no
deflection at all, and this stays the same all the way to 14349.
So, I guess this tells me the ohmic loss in the shack lash up is
around 0.1 to 0.2 dB increasing with frequency across the band.
I wanted a way to double check this conclusion, so I got a couple
more Alpha-Delta switches, so I can switch directly between the
LMR 400 input directly to the 1000D, or with a flick of the two
switches, run through the entire in-the-shack cable lash up of
all components mentioned above. W1AW was putting a nice
S7 signal out here this afternoon with the LMR directly onto
the D from the 20 meter vertical. Switching between the
direct path and the complete shack lash up causes absolutely
no audible nor S meter reading difference on the 1000D.
Seems to me it is pretty difficult to measure what one has in the
way of actual performance in this circumstance; and I believe it
is probably true in all our shacks when the antenna, the transmission
line, and all other goodies in the path are not each exactly of the
same impedance at each and every frequency of operation and
interest!! Walt Maxwell, W2DU, taught us not to worry about any
of this, so long as the rig saw a 1.0:1 match at its connection to
the system. He argues in his book, Reflections, that as long as the
rig sees a 1.0:1 vswr at its input/output port, all the transmitter
power we generate will be radiated by the antenna, and all the
energy the antenna collects will get into our receivers! I hope that
is true in my particular situation. Per Walt, this was possible because
if the rig saw 1:1 vswr, caused by whatever technique of matching
the entire antenna system was being used, the "system was conjugately
matched, and the power went through just fine, in each direction as the
reactances were cancelled away, and the "only" loss would be in the bits
of ohmic resistance in the cable, switches, couplers, etc. Just hope my
ohmic losses are really and truly low, but I am still suspicious and
puzzled about my measurements!
73, Jim, KH7M
On the Garden Island of Kauai
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|