On 4/5/2019 4:16 PM, larryj@teleport.com wrote:
Jim, is this a fan dipole that's fed with open-wire feeders?
No, my fan dipoles are all fed with coax. My primary 80/40 antenna for
JA and SA is a fan fed with RG11 (because it's up 120 ft, the feedpoint
Z is around 88 ohms). I had another one broadside to EU/VK until I
rigged a second dipole as a reflector, and made both of them 80 M
dipoles, making the directivity reversible. To make the reflector work,
I have to switch in a stub in the shack that shorts the feedpoint.
Before I found a clearing in my dense redwood forest large enough to
turn a 3-el SteppIR, I had two 3-wire fans cut for 20, 15, and 10M at
right angles to each other, also up around 100 ft. They were a better
match to 50 ohm coax, so that's what I used.
I've used my 80-Meter inverted-vee fed with ladder line (lazy man's open wire)
on all bands with a tuner (Johnson KW Matchbox, Dentron MT-3000A, etc.) for
many years (seems to get through just about every pileup, every band.)
One of the advantages of resonant dipoles is their predictable pattern,
with a single pair of broadside lobes and nulls off the ends.
Would I gain anything by actually having several "near resonant" lengths in a
multi-band fan-dipole fed by open wire, as compared to just using the 80-Meter antenna
through the tuner?
No, because resonant dipoles are a mismatch to the high Z of the open
wire or ladder line, so you still need something in the shack to match
the line to the transmitter. By the time you've done that, total loss
in the system is likely to be about the same as in good quality coax on
the bands where the antenna is resonant.
Also, as N6BT has famously said and demonstrated, "everything works,"
it's just a question of how well. His demonstration was to work all
continents with a light bulb. I have 170 countries worked with 5W in
about five years, including many DXpeditions. Many years ago, when I
lived in Chicago, I worked a guy on Long Island on 6M whose antenna was
in his basement. I think we'd all agree that we can do more more power
and better, higher antennas.
Another factor is location. If your measure is DX, defined as another
country, the midwest and east coast are a LOT closer to a lot more
countries (EU, AF, Caribbean) than the west coast, where I now live. To
work EU below 20M, I need every dB I can get, just as midwest and east
coast station do to work JA.
I know about having coax-fed (with chokes) fan dipoles, which I've used years
ago and also had good luck with those.
Another major advantage of resonant antennas fed with coax is that you
can choke them to reduce RX noise. i don't know of a practical way to
choke non-resonant antennas, or antennas fed with open wire or window line.
73, Jim K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|