[TowerTalk] Feeding half wave vertical (was Unique Inv L...Help!)

[Donald Chester]

Jerry, K4SAV wrote:

"The problem with end feeding a half wave wire on 160 (in this case
configured as an L) is that most people who operate 160 soon discover
that you need an amplifier.  That translates into big voltages at the
feedpoint.  In this case the feedpoint of the antenna at 1.87 MHz is
about 7000 ohms.  At 1500 watts that is 4600 volts peak.  So you need a
large vacuum variable.  You also need a sizable coil. You also need a
lot of margin to handle high humidity conditions.   All wires inside the
matching network have to be well insulated to prevent arc-over during
high humidity.  A little condensation is a major problem.

"It's a lot easier at 100 watts.

"But why would you go to all this trouble when a shorter antenna would
work better, plus be much easier to match?  Well there is one reason I
can think of, maybe if working DX is only a secondary consideration and
you wanted the antenna primarily for local contacts.  But if that was
the case, a dipole would beat it."


In my case, it was a quarter wave, base insulated 160m vertical, with extensive buried radial ground system (120  radials, each 133 ft. long).

I decided to see how it would work on 80m.  I used a 400 pf air variable, with 3/16" spacing (approximately 7 kv), and some 4" diameter edgewound coil stock I had in my junk collection.  The capacitor resonated at about 60% mesh.  The coax transmission line was tapped up a couple of turns from the grounded end.  Within a  matter of minutes of moving the clip around, I had found the spot where the SWR on the coax was exactly 1:1.  I loaded the transmitter up to to about 800 watts DC input, plate modulated AM.  No sign of arcing at the capacitor or corona discharge, even on modulation peaks. I used it for a short while, but didn't find much difference, transmitting or receiving, between the halfwave vertical and my 119' high dipole, except for a more pronounced skip zone, so I didn't keep it connected because I had no weatherproof shelter to protect the coil and capacitor in the tuning unit.  Never got any DX reports camparing the two, but listening to some of the SWBC stations on the top end of 75 that come in from Europe during the evening, reception was about the same for both antennas, both for signal strength and s/n ratio.

I also have used the end-fed zepp configuration for a half wave horizontal antenna, and corona discharge or arcing were never a problem, even at high power.  Nor have I ever had any problem with corona discharge at the ends of a half wave dipole, no matter how high the power.

I don't see how there would be much difference between rf voltages developed when feeding a halfwave vertical directly at the base, compared to feeding a voltage loop at the end of an open wire tuned feeder to a dipole, where parallel tuning with a balanced, link-coupled tuner is used.

The only time I ever had any kind of arcing problem due to excessive rf voltages with a parallel tuned ATU was when I would try to load into a highly reactive load, particularly an odd eighth wavelength lying midway between a current and voltage loop on the feedline or vertical radiator, by tapping down on the coil. I once tried that and achieved a perfect 1:1 match to the transmission line, but the tuner would arc over on modulation peaks at 100 watts, even though I was using a 7 kv variable capacitor.

Also, if the rf voltage at the end of a resonant antenna were that much of a problem, wouldn't the top of a vertical antenna tend to discharge like a Tesla coil, particularly at AM broadcast stations running 50 kw?


Don k4kyv




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