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Author Topic: Shack to Antennas only 45' RF back into the Shack ??  (Read 1649 times)
WN4E
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Posts: 9




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« on: March 13, 2013, 07:36:32 AM »

I have read several posts but none exactly fit my situation.  Getting ready to plant my tower and mount multiple VHF/UHF beams.  Trying to keep feed lines as short as possible due to losses.  The tower will be about 10' from my station. Since I live on a hill I don't need excessive tower height so looking to only put up 45', then using a 24' mast pipe will put most antennas 50-60' up. My station is around 10' above the ground level, so this makes me around 45-50' away from beams.  I run 1400w on six, 1 kw on 2mtr, 500 on 222 and 500 on 432. When I have my beams turned anywhere between N & E the booms will be directly overhead and up at the 45-50' level above me. Will I get transmitted RF energy back coupled into my gear due to the signal strength? Always had tower 60-70' away and 100' up to beams so never worried about the transmitted power. Now trying to reduce feed-line losses.
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KA4POL
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« Reply #1 on: March 13, 2013, 08:00:22 AM »

At these ratings there is a high probability of RF getting into your equipment. So you need to make everything as RF tight as you can.
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WN4E
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« Reply #2 on: March 13, 2013, 08:05:41 AM »

All coax's in the shack are double sheilded and have a ground to each component, granted it is probably just a DC ground and not a great RF ground.  So maybe someone will chime in on how to solidify a good RF ground.
Tnx
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KA4POL
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« Reply #3 on: March 13, 2013, 08:15:49 AM »

As a starting point see: http://www.w8ji.com/rfi_rf_grounding.htm
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K5LXP
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« Reply #4 on: March 13, 2013, 08:29:31 AM »


The antennas you're using will have impact to the incident field strength.  At V/U with beams, most of your energy will end up towards the horizon and not straight down.  So most of the typical "RF in the shack" problems as you'd have with HF and common mode aren't there.

The next question is susceptibility - how much energy, and what ingress points is your equipment most sensitive?  You likely won't discover them until you try it.

An "RF ground" isn't some connection point where all energy just disappears.  Especially at V/U, consider the entire station "floating".  Your RFI mitigation options are shielding, choking and bypass.  Until you identify a specific problem I wouldn't worry about it.


Mark K5LXP
Albuquerque, NM
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AA4PB
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« Reply #5 on: March 13, 2013, 08:35:20 AM »

"An "RF ground" isn't some connection point where all energy just disappears."

Oh no  Grin I though an 8-foot ground rod always soaked up all the excess RF. Just don't use two of them because then you have the infamous "ground loop" where all sorts of bad things happen.

Sorry, couldn't resist.
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WB2WIK
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« Reply #6 on: March 13, 2013, 10:34:13 AM »

You'll never achieve a real RF ground at VHF-UHF, so even trying seems silly to me.

I've run 1500W output stations on VHF-UHF for years and years to highly directional beams very close to the station and never had any problems of any kind.

Some very sensitive circuits like external CMOS keyers and such can glitch from strong RF fields, but they're pretty easy to fix.

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WN4E
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« Reply #7 on: March 13, 2013, 11:13:15 AM »

You'll never achieve a real RF ground at VHF-UHF, so even trying seems silly to me.

I've run 1500W output stations on VHF-UHF for years and years to highly directional beams very close to the station and never had any problems of any kind.

Some very sensitive circuits like external CMOS keyers and such can glitch from strong RF fields, but they're pretty easy to fix.


This is what I was hopeing to find out.  Someone who has done it and has hands on results.  My thanks to you.  Like I said earlier, i have always ran this station but antennas where better than 100' away.  So this time I am shooting for no transmission line losses( minimum) thus requiring a lower and closer antenna.  maybe catch you on a opening someday.  My antennas are all M2 inc.  I did scale down from 4 antennas on each band with a H-frame to now just stacked on a single mast.  7el 6mtr, 2-2m8wl's, 2 2227wl's, & 2 43213wl's.
Tnx Again
WN4E
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WB2WIK
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« Reply #8 on: March 13, 2013, 02:35:34 PM »

You'll never achieve a real RF ground at VHF-UHF, so even trying seems silly to me.

I've run 1500W output stations on VHF-UHF for years and years to highly directional beams very close to the station and never had any problems of any kind.

Some very sensitive circuits like external CMOS keyers and such can glitch from strong RF fields, but they're pretty easy to fix.


This is what I was hopeing to find out.  Someone who has done it and has hands on results.  My thanks to you.  Like I said earlier, i have always ran this station but antennas where better than 100' away.  So this time I am shooting for no transmission line losses( minimum) thus requiring a lower and closer antenna.  maybe catch you on a opening someday.  My antennas are all M2 inc.  I did scale down from 4 antennas on each band with a H-frame to now just stacked on a single mast.  7el 6mtr, 2-2m8wl's, 2 2227wl's, & 2 43213wl's.
Tnx Again
WN4E

If you can, check out CQ Magazine, April 1985 issue for photographic details of what my station was back then in NJ: 1kW output or more on 50/144/220 MHz, 800W on 432, 200W on 902 and 200W on 1296 all with antennas directly over the station to keep the lines short: And the lines were all Heliax.  The "then" station is pictured as well as antennas.

I lived on a hilltop at that time with no tall trees, so my VHF-UHF antennas were only up 35-40 feet above ground and that's about all they needed to be, as my visual horizon was 30 miles or more in almost all directions.

Zero interference of any kind to anything.  Although lightning would occasionally ZAP the masthead GaAsFET preamps. Sad
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WA3SKN
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« Reply #9 on: March 14, 2013, 04:51:17 AM »

"Will I get transmitted RF energy back coupled into my gear due to the signal strength? "
Probably not.  RF comes back when not terminated properly or a balanced-to-unbalanced" situation exists.  The beams will keep the RF going in the right direction.
Of course, a survey should be done whenever working with high power VHF/UHF.
73s.

-Mike.
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WN4E
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Posts: 9




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« Reply #10 on: March 14, 2013, 02:28:37 PM »

Many THANKS again to you guys.  I did not expect any problems but thought I would ask up front so I could divert or plan around this situation. I have 80' of tower but only wanted 45', enough to clear the roof-line by a few feet(10). On my previous tower of 100' I ran 7/8" hard line for all bands.  This shorter install I will cut the 7/8" to apprx 50' lengths this will allow for short rotor &  phasing lines up top and 8' jumpers from outside window to back of meters.

Again Many Thanks and hope to work you when installation is completed.
O.C. 
WN4E
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