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Author Topic: New to Verticals  (Read 601 times)
K1HMS
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Posts: 467




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« on: December 06, 2017, 04:13:43 PM »

I know a lot about dishes/feeds, beams, wire dipoles and Delta Loops (HPOL). I'm venturing off into verticals.

I have a 33' tall vertical mounted on the top corner (12' AGL) of a 40' steel shipping container that is setting on the ground. (I didn't have a HUMVV or MRAP handy) I'm feeding it with 50 ohm coax, center conductor to the vertical and shield to the container, there is a choke. It has better than a 3:1 match on 40m and a turner gets it down to 1:1 across the band. I have good signal reports from 600 to 5500 miles away.

On 20m the turner can achieve a 1.8:1 VSWR. Based on failed QSOs attemps and RBN I'm clearly just warming the coax.

My background is EM theory and transmission lines so I know the science. What I'm asking for is practical guidance like is there a favored balun, does someone sell loading coils or do folks just wind their own, are there tricks to using such a long monopole on 20m and higher. What about a series "loading cap" above resonance where this will  look inductive?  With a quick look on the analyzer and 4NEC it looks like it is a mono-bander.

If there is a creative solution I'm sure someone here has found it.

Thanks,

Hamilton K1HMS
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WB6BYU
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Posts: 17189




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« Reply #1 on: December 06, 2017, 04:34:10 PM »

On 20m you  have an end-fed half-wave vertical.  It can work very well, but the feedpoint
impedance may be around 2000 ohms, which is why the coax losses can be significant.

There are multiple ways that you can match the antenna on both bands.

The most common commercial approach is to add a trap in the antenna somewhere so
it is only 1/4 wavelength on 20m, and gives a good match to your coax.  The gain isn't
quite as good, but that's the way most trap verticals work.

My favorite approach is a circuit that I developed years ago, only to see it published by
someone else in the next issue of QST i received a few days later.  It uses a coil and two
capacitors in a box at the feedpoint to step the 20m impedance down to 50 ohms, while
leaving the 40m match as it is.  Here is a description.

I used this approach on a couple of 40/80m verticals and it worked well, though it narrows
the SWR bandwidth a bit.  (That's more of a problem on 80m than on 40m.)  You can, of
course, just use a manual switch or jumper wire at the base to switch in the matching network
on 20m.  With this approach, no separate tuner is required in the shack.

An approach that I've seen more often recently is to use an autotransformer (4 : 1 or 9 : 1)
at the feedpoint and a tuner in the shack.  The purpose of the transformer is to reduce the
impedance excursions on the antenna, but it really works better if the antenna is shortened
a few feet to reduce the high impedance on 20m.  You do need a tuner in the shack, however,
and the coax losses are still somewhat higher.

Or put a cross-bar on the vertical and use it to support a quarter wave wire for 20m in parallel
with the 40m vertical.  You can have it stick out both sides and add one for 17m as well.


There are more, but it should give you some ideas.  My preference is the dual-band matching
network, as then you are using the full height of the vertical on 20m.
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KM1H
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Posts: 2629




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« Reply #2 on: December 06, 2017, 04:52:28 PM »

Im not a fan of traps so offer this:

Cut the antenna at the 16' point and reconnect with an insulator. Use a 24VDC surplus vacuum relay to connect the sections together for 40. Use RG-6 quad shield spaced a foot away to feed the control voltage and decouple well with a couple of 43 mix toroids and a .01 bypass cap.

Add a small top hat for 40 so it resonates properly.

Carl
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HAMHOCK75
Member

Posts: 396




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« Reply #3 on: December 06, 2017, 05:26:36 PM »

I am envious that you have a full size 1/4 wave monopole. I wouldn't disturb it at all. How about just add another 20M monopole which is only 17' tall on the same roof?
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K1HMS
Member

Posts: 467




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« Reply #4 on: December 06, 2017, 08:08:11 PM »

Great! All 3 responses are what I was looking for.

WB6BYU - I will try the approach from the QST article first.

KM1H - The antenna is a standard US Military model and is made up of 8 4' screw together sections. It would be easy to make a insulator to screw in between sec 4 and 5.

I'll look at powering the relay with a wire broken into short sections with old Miller RF chokes right up the antenna. I have seen a system that used wires broken by small caps with a larger cap and rectifier diode at a latching relay. A series of fast rising short pulses were used to toggle the relay. The switch wires are transparent at HF.

Hamhock - A variation of your idea is another of the same antenna with just the 20m L section  from the QST article. I'd swap out the top section with something a bit shorter to reduce the high Z at the feed point.

73
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WB6BYU
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Posts: 17189




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« Reply #5 on: December 07, 2017, 08:38:39 AM »

Quote from: K1HMS

...I'd swap out the top section with something a bit shorter to reduce the high Z at the feed point.




No need to - that circuit is designed to match the high impedance of an end-fed half-wave whip.
That's why it is handy to use on both bands, as no change is needed at the antenna.

Some of the old variable capacitors from ARC-5 transmitters were designed to be locked into
place at a particular setting, and didn't have shafts.  They are often cheap at ham fests because,
lacking shafts, they don't work as well for variable capacitors.  But they should work well in this
application.

In one of my early versions I used a variable capacitor for the series capacitor, then measured the
value and replaced it with a tubular ceramic cap (from the 1950's?).  That cap blew after a few
weeks at 50W - it was probably designed for use in a tube receiver at up to 250V or so..  I might
try a higher voltage type, but an air variable or mica compression trimmer is probably a better
approach.

The exact values depend on the exact installation, so expect them to vary during initial adjustment,
but it isn't difficult to set it up with just an SWR meter:  vary the series capacitor for best match on
40m, switch to 20m, adjust the shunt cap, and see what the SWR looks like.  If necessary, move the
coil tap and repeat the process until you get a good match on both bands.
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KM1H
Member

Posts: 2629




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« Reply #6 on: December 07, 2017, 06:04:48 PM »

An end fed vertical half wave on 20M is pretty useless.

He is already up in the air so using 4 1/4 wave elevated radials for each band gives him a good low angle signal on both bands.

He can also leave the existing vertical alone except add some aluminum or a top hat to resonate.

Then run a 20M wire up the side spaced 4-6".

No tuner need, no restricted bandwidth with the SS amp, and no funky radiation pattern.

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