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Discussion in 'CB Antennas' started by Mudfoot, Mar 25, 2020.
Are these subject to common mode like the Antron and Imax?
having radials is no guarantee that you won't have cmc, you can be unlucky with mast or coax length & have cmc with any of them,
imho having proper radials lessens the chance you will have cmc that causes issues,
we just took a locals spt500 down & replaced it with a no radial IMAX 2000,
there was bad rfi in the shack that was not there at levels to cause rfi with the spt500,
with his mast & coax length there was a significant increase in rfi, it was unusable with his amplifier, that's where most people start shouting POS imax fckin fiberglass crap,
adding radials & isolating the IMAX fixed the rfi,
any of them can work well when the mast & coax are not radiating, even a99 & imax.
I've had issues with prior Antron and Imax. Both had radial, but had RFI bad. My Gainmaster didn't. It's getting old and I want to replace it with a Penetrator.
The radial kit alone won't fix cmc on the imax or a99,
you should be ok with the spt500, if you have any issues, isolate it from mast & choke the coax.
I had rfi issues with my old IMAX at first. I added a choke at the feed point and put a Bencher Ya1 lowpass filter in line. Took care of my issues at 500 watts.. Also going from 30ft to 62 ft at feed point helped also..
I have a current balun. I was saving it for a different project. Thinking of using it on the SPT-500. Once I get this up, I don't want to take it down. It's very difficult erecting it where it's going. Do you think I will see a significant improvement over the Gainmaster?
no harm in using the balun at the feed-point, 5 turns of rg213 on a 4.25" former works well too,
can't say if you will see much difference between gain-master & spt500,
gain-masters work great if they are up well in the clear,
in my gain-master vs i-10k test not high above ground the gain-master was close to the 10k on the same pole, notably closer than the no radial imax on same pole,
gainmaster edges ahead of sirio 827 in our test too,
i have since realised that MOST of the differences people report about antennas are caused by the mast & coax or ground wires radiating, not by any special property of the antenna,
the differences seen from lower loss matching is small,
difference due to not having the mast/coax radiating can be large.
I suppose I should just keep the GM. I've had it since they hit the market. I had an IMAX and Antron before it. I dealt with neighbor complaints with both of those. The GM is problem free in that regards.
It handles my 811 and ALS-600 at full output on SSB and CW. I keep CW output around 500 watts. lt even survived a Derecho storm event 6 years ago. It was one hell of a sustained high wind. I think I'll lower it to inspect and install new coax and be done with it. Put my money towards something else.
I didn't need and fancy equipment to realize my GM outperformed the IMAX and Antron. I had a near full S-unit increase.
I was a little curious about the Penetrator and how it acted on 20 meters.
I'll see how much life the GM will give me.
Mudfoot, if I could still work my radio and what goes with it, I would have a GainMaster. I'd buy 2. Your report is you own best advice.
I use to love the idea for the SP500, but I've seen and heard too many bad reports about them being weak, with insulator issues, and the recent version's differences in construction. Other than that, I think the SP500 design is probably about as good as you'll get with a 5/8 wave CB antenna.
Good idea to replace the coax. Before you go up, you might consider comparing the old and new coax lines for watts, through-put, using a good meter and a good dummy load. I always found this test revealing.
Good luck and keep us posted.
The coax is in good condition. I used quality 8x. It's been cobbled where I ran it in different directions over the years, but want to refresh it. I have some nice LMR-240UF I picked up couple years ago. Just got lazy and put it off. Recently, both my wires came down due to rotting cord or darned squirrel. Not sure, but it looked chewed. I took that opportunity to replace with new. Just have the GM left to re-coax.
I scanned my GM with binoculars today. It looks good from the eye. I might just order a new GM. It's very difficult to lower my mast, especially at my age. Just want to do it once. I always use my analyzer. I'll have a 100' run to the GM.
I am curious why you used 8x coax instead of 213 or similar coax on a 100 foot run?
I guess I should clarify. The old 8x is 65'. The LMR-240 will be a 100' run to allow enough length, if i decide to switch to a different location within the house. I
Got it, thanks.
if i had a gain-master up & it was holding up to wind & the power i wanted to use,
i would not think about swapping it for another 5/8wave, you aint missing anything.
Wonder if they'll ever improve it? More power handling and mechanical strength would be nice.