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Author Topic: How To Test An Old Hy-Gain Vertical Trap Antenna?  (Read 1926 times)
N5KNG
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Posts: 103




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« on: April 24, 2002, 11:42:42 AM »

I have inherited an older version of the 14AVQ/WB 40m 20m 15m 10m trap vertical antenna, which has been sitting out in the rain and other elements for 15 years or so.  It wasn't connected to anything - just lying there.

Are there any simple tests I can do (other than just hooking it up to my radio and praying there is no dead short while I apply transmit power and measure SWR) to determine if the capacitors/inductors in the traps have survived weather, tension/torsion, and time?  I am afraid to disassemble it, because I don't want to break anything inside the traps.  Each section appears to be fairly well "welded" to the next with some amount of rust.  Some of the plastic in the covers on the lower part of the traps appears to contain holes, as do the rubber covers on top of the traps.  Most of these holes are due to wear over time, and not by design.

What about the idea of hooking it up for receive only, and listening to noise over each band to see if it peaks anywhere near where resonance is supposed to be (I do have the manual for it, and it tells me how the antenna was assembled, along with what the SWR vs. frequency should be in each band)?
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WB2WIK
Member

Posts: 20613




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« Reply #1 on: April 24, 2002, 12:01:28 PM »

After 15 years of "outdoor" storage, if it were me, I'd use a tubing cutter to cut the antenna into pieces small enough to fit my recycling can and toss the works.

However, if you like a challenge (!), surely set it up and try it out on receive.  If it hears well, it's neither an open nor a short circuit and that's a good start.  If you use 100' of intentionally lossy coax like RG58/U (which I wouldn't recommend for a real installation), even if the antenna is horrible, you shouldn't do your transmitter any harm because the cable loss is sufficient to form a reasonably good 50 Ohm load -- even without an antenna connected to the far end.

The evidence of rust (?) and holes in the trap covers concerns me.  Aluminum shouldn't rust, but it may be well oxidized.  Possibly the antenna is so old that it was assembled with plated steel hardware and not stainless steel, so rust from the screws and nuts, etc, has formed and run along the aluminum parts when it's rainy, and badly discolored the aluminum.  The trap covers, at least the "upper" ones that would face the sky when the antenna is vertical, will need to be replaced to keep water out of them.

The holes in the trap covers may indicate the presence of insects within the traps.  That won't necessarily ruin the traps or the antenna, but it could be a scary discovery if you tilt the thing upright and have black widows crawling around (a frequent problem where I live, in southern California).

A brand new 14AVQ from Hy-Gain (they're still made, today) costs $159, not a bad deal.  Because they're still on the market (MFJ resurrected Hy-Gain by acquiring them), I imagine spare parts are available as well.

Good luck!

73 de Steve, WB2WIK/6
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WB6BYU
Member

Posts: 13353




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« Reply #2 on: April 24, 2002, 12:01:51 PM »

You can start by measuring the DC resistance across each trap.
Should show very low resistance - if not, there is a broken wire
somewhere.  (You may have to scrape off some aluminum oxide to
get a good contact for the meter.)

If you measure the DC resistance across the feedpoint, you
SHOULD see a low resistance - if I recall, there is a shunt coil
to ground to drain static charges, and to provide some impedance
matching on the lower bands.  So don't worry about it.

I suggest you just install the antenna.  If you can borrow an SWR
analyzer, you can quickly check the SWR on all bands (and find
resonances outside the bands if they have shifted.)  Presuming
you have a 100 watt rig, you can turn the power down to 1 to 5
watts and measure the SWR with no worries about damaging the
finals if the SWR is high.  (Some of the CB-style SWR meters are
frequency sensitive, and require more power to drive them to full
scale on 80 or 40m than on 10m, but you still should be able to
get a reasonable idea whether the SWR is "too high" or "ok".)

Remember, of course, that the ground and radials will affect the
resonant frequency and impedance of the antenna.  If you install it
temporarily, the resonant frequencies may not be the same as they
will be when it is permanently mounted with a full set of radials.
But it should give you a reasonable idea of whether or not the
antenna is working.
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KD5FOY
Member

Posts: 40




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« Reply #3 on: April 24, 2002, 12:02:14 PM »


get access to an mfj antenna analyzer or an autek
analyzer.  i'd bet someone in your hamclub has one
they'd loan or whatever

larry
kd5foy
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KC5NIS
Member

Posts: 26




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« Reply #4 on: April 24, 2002, 10:26:29 PM »

The center conductor on the antenna base will read a dc short to ground.  There is an inductor inside the black base connecting the radiator to ground.  If the traps still have good connections inside them and there are no bugs (real bugs), then the antenna should work fine.  The design of this antenna has not changed hardly at all so parts are still available.  Those plastic caps can be replace for about 75 cents each.  The antenna can be repaired no matter how bad it is.  It just depends on how much fun you will have and how much you will learn fixing it.  I love fixing gimme stuff even if I don't need it myself.  Hygain still has parts  662-323-9538  there is a 800 number but I can't find it.
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