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SWR Too low

M

Mark Malcomb

Well-Known Member
Nov 27, 2019
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I just ran 130 FT. of Tram RG8X to my GM antenna. When I checked my SWR it barely moved the needle.
It was low with a hundred feet of crap cable. ( 1.7 on ch 40) Now i'm concerned if this coax I just bought is junk. And this is dangerous to my XMIT on my radio.
I will learn some day with help...................Mark
 

It is entirely possible to have good swr on a long run of coax with nothing connected at the other end. It doesn't necessarily mean that the cable is bad. It just means that the 50 ohm cable is a big enough load to not reflect any back to the source.
 
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OK, Buddy, I apologize, Does that mean its good or bad? It is connected to my GM
Which by the way this forum informed me a dead short between center and shield is
accurate. Wow it freaked me out till I got that confirmed. Not sure where or if I check 50 ohms on one of cable while hooked to antenna if shield and center are common. I may have a great swr with an extra 30 feet added.........Thanks...Mark
 
It is, Thanks didn't want to fry my radios. I guess its good coax. Thought it would go higher when I added 30+ feet. I appreciate the help..M
 
BUT...

IF you are using a A99 or other SHUNT FEED antenna ...

Read this as a DC Short...meaning it uses a BALUN ACROSS CENTER TO SHIELD - it will read as a dead short.

There may be a coil (matching network inside the antenna) that takes the 50ohm coax and "converts it" or Transforms it - to the higher impedance the antenna is - say if it's a 1/2 Wavelength End-Fed - Like an A99, then the impedance at the FEEDPOINT is very high, into the Kilo-ohm range. 1,000 X times higher than 50 the 50 ohms coax can easily match to. So the Balun is a coil - depending on the antenna - UL Lists, as well as other LOCAL LAWS and REGULATIONS - require many BASE antennas to be DC-Grounded - so in case of lightning strikes or other issues with Ground potentials - the antenna wont attempts to send back excessive hazardous voltages back to you and the radio.
AntennaBalun.jpg​

So get to the antenna - and find that connector - measure with your ohm-meter (DVM) and get the ohmic value ACROSS Shield to Center (back and forth across sort of thing) AT AND IN THE ANTENNA SIDE OF THE CONNECTOR - copy write down that reading - and ALSO THE COAX THAT CONNECTS TO THE ANTENNA - make sure you don't have that short in the coax - like it's been pinched...to see if it is a true short in the coax. IF not, then reconnect the coax then run back to where you have the coax going to the radio - and measure that - any ohmic difference is a loss - so try to keep the difference as close to zero ohms from one end to the antenna.

Then for other SAFETY reasons, - measure the SHIELD to GROUND at the station - your Station Ground - don't have one? Better to play it safe and make one to protect you and your equipment. Ground at Station should be at or closest to Ground potential it can be to keep power currents from hurting you or your station. Lower the OHMIC readings on your DVM the better - less potential differences keeps the power from possibly flowing back into your station and using it as a means to go elsewhere - including you.
 
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Many antennas will read as a dead short when measured with an Ohmmeter.
I had a long run lf RG-213 attached to my radio and did not have anything connected to the antenna end. Perfect SWR. Cable makes a good low SWR load if the length is right.
Remember the only reason to worry about SWR is to protect the finals in your radio/amp. Low SWR does not mean your antenna is resonant.
 
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The old cable came with the Gain Master when I just bought it. It was a freebie. It was made in Italy Like the antenna. The antenna has the balum you mentioned. The coax was not long enough for my set up. Short by 30 feet. However this great site put me at ease when I was told this antenna measures dead short from shield to center at antenna. I haven't actually erected it up yet, it's just 4 feet high now. I was gona jack it up to 15 feet this weekend. That may be all I can do in this bloody HOA I live in. SCREW the neighbors. LOL. Thanks for your help Andy & Homer....M
 
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!!##@ ??? Very sorry Mike that just blew over my head.. However you told me my GM reads dead short across center to shield.........Cool.....M
 

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