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Antenna Specialists M-117 base antenna

Mp1969

New Member
Jul 12, 2017
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i was given the m-117 magnum base antenna. I believe it to be a 1/2 wave antenna. I put it up only about 15' above the ground. It seemed to receive ok but the swr was the same across all channels over 3. Adjusting the height had little impact. Testing the continuity on the connector at the base, I find that there is continuity across the connector. I mean the inside to the outside of what looks like an so-239 connector.
To me this means a short. From what I've been able to dig up on the internet, it may not be. It could be this way by design.
Does anybody have any experience in this matter? Is there a short or do I just need to tune it? I am new and eager to learn.
Thank you in advance.
 

you should have a short when measuring with a multimeter on the s0239 connector, its a shunt fed 1/2wave,
magnums were prone to rain getting into the coil cover & causing high vswr,
sometimes freezing & busting the cover, try cleaning the joints where the tubes slot together, if its still over 3:1 its time to get into the coil housing & see what's up.

good luck
 
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Thanks for the response. I have the antenna isolated from the mast right now. Does it need to be grounded to function?
 
ground the antenna to a metal mast and try a different coax, one that is known to be good,,,,and try again,,, are you using an external meter and not the swr function on a radio?
 
Yes. I am using an external meter. I am going to clean all the connections and see where it's at.
Thanks again.
 
if you still get a high reading then the small coil inside of it may be burned up,,, common whne you use more than 300 watts thru them,,,,
 
Never underestimate how much water can leak into the plastic coil housing.

The short circuit your meter sees on the meter is correct. This antenna is fed from the coax directly into a tap point partway up a coil. The top end of the coil feeds the bottom of the vertical element. The bottom of the coil is grounded. RF will see 50 ohms when it's working right.

My best guess would be weather exposure and corrosion inside that plastic coil housing.

73
 
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Thanks for the great information. All external connections are cleaned and still high swr reading.
I am going to get inside the coil housing as 'delicately' as I can and see what's going on. Thanks again for the much needed input.
 
Well the plastic housing came off easily. Lots of corrosion underneath. The small copper wire coil is in tact though and I cleaned its connection to the bottom of the vertical element. Looks to be about 14 gauge wire. I will give it a go tomorrow.
While I was in there, I thought about replacing the wire coil. Maybe with a larger or smaller gauge wire? Maybe make it longer or shorter? Would that have any effect on the antenna? Is this a way to tune it? Does the number of wraps before the point of contact matter? So many questions.
 
Yo Mp1969: Yeah the coil length and turns and diameter and contact points all matter, as this is all calculated into matching the coax to the 1/2 wave length antenna. Clean it all up and apply RTV Silicon Rubber to the SO-239 connector were the wire is soldered.

The coil can be replaced with the same 14 gauge wire, length, diameter, and contact points on the coil. I won one of these Magnum Antennas several years ago, and in a need to get on the air one night after a few beers I put up the Magnum Antenna. It will take ball park around 1000 watts, but I wouldn't push it any more in power.

Mine broke after a massive Santa Ana winds clocked at 80MPH plus, the bottom Plastic tube broke. I reassembled the bottom plastic tube with epoxy, and reinforced it with fiberglass cloth, and resin, several layers, and it was fixed and much stronger. Good luck.

Jay in the Great Mojave Desert
 
I have two of them. Ones NOS and the other i got from a ham. The ham kept one and sold me one. The one he uses he rebuilt with heavier wire and he normally runs at least 1 KW into it on SSB with no issues on 10 meters. But like another poster said it doesn't like moisture and it will react adversely to moisture and you'll be greeted with a bad swr.
 
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I have two of them. Ones NOS and the other i got from a ham. The ham kept one and sold me one. The one he uses he rebuilt with heavier wire and he normally runs at least 1 KW into it on SSB with no issues on 10 meters. But like another poster said it doesn't like moisture and it will react adversely to moisture and you'll be greeted with a bad swr.
I'd spray it liberally with Flexseal and see if that doesn't help prevent water ingress.
 
if worse come to worse you can gut out the stock coil and mod it to use a tapped coil or loop .
 
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