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HIGH GAIN ¾ Antenna Problems
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HIGH GAIN ¾ Antenna Problems
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deeplist
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Post HIGH GAIN ¾ Antenna Problems Reply with quote
I recently purchased the High Gain ¾ Antenna and I have had some problems with it. I need some suggestions. First off it had an N connector on it and I needed an SO-239. So I went and got an adapter from Radio Shack so that was solved. I have a 300 watt amplifier that I used before and it works great. After putting up this antenna the SWR went through the roof. The resistance in the antenna is outrageous. I am getting over 35 watts back down the line. The amplifier will only put out about 170 watts now because of the high SWR which is about 1.6:1 right now. I slid the top elements out to where they were marked from the factory. I am not sure what I should do. The antenna becomes hot to the touch because of all the resistance.

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Post Reply with quote
1.) Adjust SWR (You've got SWR meter, use it)

2.) Antenna will always be hot to the touch when you touch it
while it is under RF voltage, especially when you're using 300W.
I would not recommend doing this actually, RF burns etc might
result.

Impedance and resistance are two entirely different things.
Something like ARRL antenna book is perfect to get you on speed
with antenna basics.

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deeplist
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Yeah, but I thought it was already tuned for the best SWR from the factory. Exactly what am I supposed to adjust? The top elements already have markings on them to show where it is supposed to be.

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Post Reply with quote
Did you specify frequency or not when you ordered it?

Anyway, email me and I'll send you instructions for tuning.

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deeplist
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I did specify, but I would still like the directions regardless.

admin@deeplist.com

Thank you.

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Post Reply with quote
One thing to consider:

1.) A SWR meter will only show correct power out
when it is loaded with proper load. Are you getting
300W when you use dummy load?

Example: A badly matched antenna can let your SWR
meter show much higher power being radiated. This depends
on the actual impedance, which can be too capacitive or too inductive.
Indicated power can be higher or lower in such cases-

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With the dummy load, I am only getting 150 watts out.

I am using the FM300PA amplifer and there is an internal adjuster labeled R2. What is that? Could it have anything to do with the problems?

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Well, we're getting closer now to the actual problem.

1.6:1 is not that bad of a SWR actually. You have to know
that this depends a bit upon actual installation of the antenna
and the same antenna will have a bit different SWR when mounted
in different locations, so tuning might still be required, if you're
really pedantic about it.

The other problem here is that your amp is just pushing 150W.
The problem here could be:
-Your power supply is not strong enough or of not sufficient voltage rating
-You're not driving it enough power.

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The input wattage is high enough to drive it. The voltage is correct, and so are the amps going into it. When I hook it up, the amp will put out the full 300 watts, and then after a short period of time it will drop down to about 150. The SWR is about 1.1:1 to begin with, and then it will rise to around 1.7:1. The reflected power is around 2½ watts to start out with and then it will rise also. I really don't know what is wrong.

I am about to sell all my gear and give up.

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Post Reply with quote
Is your dummy load rated at 300W or more?

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Yes, it is an MFJ Dummy Load. It say right on it: 300 watts dry dummy load. I am starting to wonder if this is actually an issue with the antenna once again. It is almost as the equipment begins to heat up it will shut down. I didn't receive any kind of documentation on the 300 watt amp, and I am still wondering what the R2 does inside when I turn it with a screw driver.

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Post Reply with quote
It might be that the power supply voltage sags (falls) after the power
supply gets warm.

Or the dummy load heats up and changes impedance.

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Post Reply with quote
I really do not believe that is the case. I am putting exactly 1 watt into the amplifer as it is supposed to take. When I first turn everything on, it all works great.

1 watt in, 300 watts out. Reflected power 2 watts, SWR 1.2:1

Then after about 5 minuites, something happens

1 watt in, 150 watts out. Reflected power 15 watts SWR 1.6:1


I can actually sit there and watch as the readings climb after about 5 minuites. I can not test these on the dummy load because at 300 watts it is only rated for 40 seconds of testing.

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In addition, the amplifer needs 52 volts and 7.5 amps as it is stamped on the unit itself. I tested the power supply with a multimeter after severl minutes of opperation and it is putting out 54.4 volts as it should be so I know it is not an issue with the power supply.

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Post Reply with quote
I don't think antenna degrades with time, it is likely that
something happens to the amplifier or transmitter.

Measure power supply voltage with time.

Amplifier might be shutting down automatically as it warms up,
try to provide better cooling to the amp.

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Alright, I have made another observation. There is an internal LED inside. I have been watching the meters and performance over a period of about 15 minutes.

The amplifer will work properly, and then suddenly the LED will shut off and the performance becomes poor. After a short while it will begin working again.

If this is in fact a cooling issue, then what can I do? There is already the fan on it that was supplied and it is working properly. What else can I do. I have ran out of ideas on this one.

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Use an extra fan, this time make it blow over the top from the side.
Or place it directly over the fet and make it blow down.

300W is a lot of power, that single fan might be struggling to get the
job done. Ideally these amps are placed in an enclosure with
one fan on both sides blowing from the sides of a so called tunnel.

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I do have the unit in an enclosure. It is very professional with a switch and an LED on the outer case, but I have taken that off for testing purposes. Should the amp be mounted with the cooling fins up or down? Right now I have them facing down with the bottom fan sucking air in from under the unit. There is about an inch between the bottom of the enclosure and the table so there is plenty of space for air to get in. The enclosure has holes in the bottom and top.

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Post Reply with quote
In every case I guess you need more cooling, work
in that direction.

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Marko - PCS Electronics
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Actually I have a 20 watt amplifier that I use for testing. I hooked that up and went out for a test drive. It sounded absolutely terrible after I got about one mile away. So I went back home and put up my old comet antenna and then it sounded great.

I am really not as satisfied with this antenna as I thought I would be. Besides the fact that it was not shipped very well and the top circle was bent upon its arrival.

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