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Author Topic: Off Center Fed Dipole and noise  (Read 6306 times)
KB6HRT
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Posts: 326




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« Reply #15 on: April 21, 2017, 02:17:36 PM »

Bruce,
When you get your antenna sorted out the way you want it!  Hear the thing I have done for a very long time, I tune the receive to my liking, by changing the leanth or the feed line, it has an effect on your receive, I presently am using 8 antennas at this QTH the longest uses 150' of  RG213u another is 70' of RG8x  2 using 100' or RG8U and another is using 100' RG8X and my 2-440m uses 100' of LMR400.
could I feed these antennas with less coax of course. so why do I do it, because it in most cases it gives me a better signal to noise ratio, an all my antennas work well an the station hears well! an I am a happy camper most of the time, also the radio see's a lower SWR which can broaded the match the radio see's..................73s..............kb6hrt

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KE6EE
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« Reply #16 on: April 21, 2017, 02:30:10 PM »


Changing the leanth or the feed line...because it in most cases it gives me a better signal to noise ratio...also the radio see's a lower SWR which can broaded the match the radio see's

Better performance with longer coax or simply more loss, which reduces both noise and SWRs?
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KX2T
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« Reply #17 on: April 21, 2017, 02:39:07 PM »

Not always so, some antennas like an 80 M OCF dipole or for that mater any 80 m dipole seem to perform better on the RX side with an electrical half wave of coax feeding it plus the benefit of lower swr on the feed line seems to be achieved. On the higher band like 20,15 or 10 meters it might show a little less loss having under 100' runs but having better coax with improved shielding seem to help allot more like LMR400 or other like coax.
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KE6EE
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« Reply #18 on: April 21, 2017, 04:33:05 PM »

Some antennas like an 80 M OCF dipole or for that mater any 80 m dipole seem to perform better on the RX side with an electrical half wave of coax feeding it.

Sounds like antenna effect due to common mode pickup, feedline acting like part of the antenna. "Improved SWR" from longer feedline means more loss. Halfwave of coax feed usually means no SWR transformation.

Otherwise, I'm not much of a believer in magic.

« Last Edit: April 21, 2017, 04:37:11 PM by KE6EE » Logged
W9IQ
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Posts: 2825




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« Reply #19 on: April 21, 2017, 04:38:52 PM »

Quote
could I feed these antennas with less coax of course. so why do I do it, because it in most cases it gives me a better signal to noise ratio

Your long lengths of feedline are simply introducing losses that equally reduces received noise and signals. You could save substantial money by simply using coax with more loss or learning how to build attenuation networks.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KB6HRT
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Posts: 326




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« Reply #20 on: April 21, 2017, 10:21:09 PM »

 Not trying to start anything, I'm saying over the years of being Old School 13 wpm General I do tuning work that needed to be done to get the results I'm looking for, it is worth it for me on my small lot.  You can use the extra coax in many ways making a choke or chokes on eather end of the coax, not having to add connectors or loss in the circuit, also you can change the leanth a little sometimes to drop the SWR, or sometimes raise a G5RV apex so it preformes as good as a dipole or a DB. If one wants a little more transmit on a give freq, or raise the apex to compinsate for any receive loss to suits your needs. I still do many hours of A/B testing to hear what worked best for my poor old ears an can so I listen longer..........Just some of what I have learned over time an was willing to do the work required which has always been FUN for me.  I have always love HAM radio,  I try an pass on some of the stuff that has worked, as my Elmer did for me, that guy could transmit & receive 60 wpm, Me barey 13 wpm, Over the past months people where my QTH is have been complaining obout weaks signals in there receive, I having 2or3  antennas for 160-75-40m has made the differance at this station on pulling those weak signals at this QTH so my work was worth it for me, an everybody skill levels in life is always changing an different..............73s........kb6hrt
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KE6EE
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« Reply #21 on: April 22, 2017, 10:21:59 AM »

I'm saying over the year I do tuning work that needed to be done to get the results I'm looking for, it is worth it for me on my small lot.  You can use the extra coax in many ways...

Of course it is great fun to try different things and to see what seems to work best. We all do this.

Lots of things work surprisingly well. Mobile antennas. Small miracle antennas which have been available commercially for decades.

On the other hand, some of us like to figure out why something works and even whether something actually works better than something else.

Figuring things out means, for many of us, understanding the scientific principles involved and trying to make measurements so as to provide evidence that our impressions regarding performance are accurate.

There simply is no performance benefit to using longer lengths of feedline, of whatever type, unless a specific length provides a high-current point at which to connect a tuner or transceiver. Longer feedlines mean less efficiency and more loss whether or not the noise level decreases or the SWR improves.

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W4OP
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« Reply #22 on: April 22, 2017, 11:20:38 AM »

If changing the length of your coax varies the SWR or changes noise, then you MUST have common mode issues on the coax. Neglecting small differences in loss vs  length, SWR is a constant along the transmission line. Pretty easy to prove with a Smith Chart, or any of the on-line transmission line calculators. It isn't magic- it's math.

Dale W4OP
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KB6HRT
Member

Posts: 326




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« Reply #23 on: April 23, 2017, 12:09:35 PM »

Bruce,
  There are so many factors in making an antenna system work at a give place to ones satisfaction, one can't name all of them, as you can read everybody has there opinion, as do I,  one thing I know is mother nature is the biggest player in this ball game, the time of the year, time of the day or night, how good or bad conditions are, so on, an so on,  we all are confronted with the same theory, your antennas, installations, types of antennas how high or low the antenna is how much power you think you may need, type of radio equipment you use, it goes on an on an thats what makes HAM radio such a fun hobby, I know very little about
theory these days,  I use basic antennas that have been proven before I was born an work if installed correctly, is there better stuff out there, you bet, I knew a lot more theory when I was a young HAM but is not the case now, because it was not reinforced because it had never served me well when I did, always had to work out the bugs to get the antenna to work well as the starting point, which requires  some tuning anyway. most of the time it was something different as it ended up, unless I was doing a duplication of something I had built an installed before. have healed on to what has worked  for me best as I remember. When I was young and going to school, there was BLACK & WHITE in my middle years there was BLACK GRAY an WHITE now theres now theres Black Gra-a-a-a-a-a-ay & White and I am not sure about that sometimes hmmmmmm. So what does all this extra work give ya maybe 5min on a Rag Chew on both ends MAYBE.
so one has to be into this stuff big time to make it pay a little for ya, so guess I am CRAZY about HAM radio.............73s.......kb6hrt
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