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Author Topic: mobile 20m ground plane for roof?  (Read 737 times)

K2JF

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mobile 20m ground plane for roof?
« on: November 17, 2020, 12:53:14 PM »

I have access to the roof of my NYC apt building at 85 feet and have an end fed up there. I want to add just a 20 meter ground plane and keep it discrete. Would there be a mobile antenna and mount I could use? I can clamp it to metal railings if need be - maybe even use the metal railings as radials? What's the simplest set up to get up there in a hurry?
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K2JF

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Re: mobile 20m ground plane for roof?
« Reply #2 on: November 17, 2020, 01:25:31 PM »

Use this to clamp to a railing: https://qsradio.com/shop-now.html#!/Claw%E2%84%A2-Mount/p/51822117

Possible Antenna: https://qsradio.com/shop-now.html#!/QuickStick%E2%84%A2-Hamstick-Single-Band-Mobile-Antennas/p/51240434/category=13335018
interesting. I like it and already do have a 20 meter ham stick. They are shortened though and would prefer a full 1/4 wave to put on that clamp mount.
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AK5B

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Re: mobile 20m ground plane for roof?
« Reply #3 on: November 17, 2020, 02:27:58 PM »

Quick and easy; MFJ-1979 17' telescoping stainless steel whip and two radials.  You can also adjust the whip for other bands and roll up the radials according to your band of operation.

73,

Jeff
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K2JF

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Re: mobile 20m ground plane for roof?
« Reply #4 on: November 17, 2020, 02:48:44 PM »

Quick and easy; MFJ-1979 17' telescoping stainless steel whip and two radials.  You can also adjust the whip for other bands and roll up the radials according to your band of operation.

73,

Jeff
what do you think about it's ability to survive in high winds?
Quick and easy; MFJ-1979 17' telescoping stainless steel whip and two radials.  You can also adjust the whip for other bands and roll up the radials according to your band of operation.

73,

Jeff
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KL7CW

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Re: mobile 20m ground plane for roof?
« Reply #5 on: November 18, 2020, 04:12:57 PM »

If you want it to be simple and work, just get the 17 foot MFJ, and clamp it to the metal railing.  If the railing is of a reasonable size and the sections are welded or screwed together for good connectivity, then if the total length is say 16 or more feet in each direction, or even something longer in one or more directions, it will probably work just fine since you can adjust the whip length for whole system resonance.  The whip may end up 15, 16, 17, 18, 19 feet or something like that.  No problem except there may be some SWR and some common mode current on the coax.  Use a common mode choke and that solves one problem, and a tuner in the shack or rig(if it is even necessary) will solve the probably small SWR problem especially if your coax loss is only something like one or so dB.  Those of us who operate portable QRP typically have  fine results using a chain link fence, metal picnic table, etc. as our counterpoise.  Is this ideal? NO, but is simple and I really doubt that you would lose more than a dB or two. The advantage is it would be easy to tune and get you on the air immediately.  If the system is satisfactory, you can always replace the whip with something like a 10 foot piece of conduit with a fiberglass bicycle or snow marker on the top and use the whip for portable operation in a park if you do not think the whip will survive long term.  If you can install 3 or 4 elevated radials each 1/4 wavelength long this would naturally be a bit better.  Another advantage of the whip, when the upper bands open up in a very few years, simply shortening the whip for system resonance on say 17 or 15 meters would be a snap. Since we do not know if the railing is well grounded to the building, or has a resistive connection, it is impossible to predict what will happen.  However I have found that perhaps 95 % of the time when I operate portable, adding any large metal object either makes no change or improves performance.  Very seldom does this degrade performance....so with the limited info I have, lets say your odds of success should be good that will be a very useable antenna. If that antenna is a "keeper" you could keep that length and put a remote automatic tuner right at the base and use it on 20, 17, 15 and even possibly on 30 meters.  If you want a shorter antenna, make it resonant on 17 meters, and the remote atu should tune it up just fine on 20, 17, 15 and possibly other bands. If you will only use the antenna on the band it is resonant on, then a tuner in the shack (if necessary) should be OK with only a little additional loss.  If you want to use the antenna on a different band than it is adjusted for, then a tuner at the base of the vertical would be preferable.  Happy experimenting     Rick  KL7CW        PS  I have never tried the MFJ whip
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K2JF

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Re: mobile 20m ground plane for roof?
« Reply #6 on: November 19, 2020, 03:42:12 AM »

Quick and easy; MFJ-1979 17' telescoping stainless steel whip and two radials.  You can also adjust the whip for other bands and roll up the radials according to your band of operation.

73,

Jeff
what do you think about it's ability to survive in high winds?
Quick and easy; MFJ-1979 17' telescoping stainless steel whip and two radials.  You can also adjust the whip for other bands and roll up the radials according to your band of operation.

73,

Jeff
If you want it to be simple and work, just get the 17 foot MFJ, and clamp it to the metal railing.  If the railing is of a reasonable size and the sections are welded or screwed together for good connectivity, then if the total length is say 16 or more feet in each direction, or even something longer in one or more directions, it will probably work just fine since you can adjust the whip length for whole system resonance.  The whip may end up 15, 16, 17, 18, 19 feet or something like that.  No problem except there may be some SWR and some common mode current on the coax.  Use a common mode choke and that solves one problem, and a tuner in the shack or rig(if it is even necessary) will solve the probably small SWR problem especially if your coax loss is only something like one or so dB.  Those of us who operate portable QRP typically have  fine results using a chain link fence, metal picnic table, etc. as our counterpoise.  Is this ideal? NO, but is simple and I really doubt that you would lose more than a dB or two. The advantage is it would be easy to tune and get you on the air immediately.  If the system is satisfactory, you can always replace the whip with something like a 10 foot piece of conduit with a fiberglass bicycle or snow marker on the top and use the whip for portable operation in a park if you do not think the whip will survive long term.  If you can install 3 or 4 elevated radials each 1/4 wavelength long this would naturally be a bit better.  Another advantage of the whip, when the upper bands open up in a very few years, simply shortening the whip for system resonance on say 17 or 15 meters would be a snap. Since we do not know if the railing is well grounded to the building, or has a resistive connection, it is impossible to predict what will happen.  However I have found that perhaps 95 % of the time when I operate portable, adding any large metal object either makes no change or improves performance.  Very seldom does this degrade performance....so with the limited info I have, lets say your odds of success should be good that will be a very useable antenna. If that antenna is a "keeper" you could keep that length and put a remote automatic tuner right at the base and use it on 20, 17, 15 and even possibly on 30 meters.  If you want a shorter antenna, make it resonant on 17 meters, and the remote atu should tune it up just fine on 20, 17, 15 and possibly other bands. If you will only use the antenna on the band it is resonant on, then a tuner in the shack (if necessary) should be OK with only a little additional loss.  If you want to use the antenna on a different band than it is adjusted for, then a tuner at the base of the vertical would be preferable.  Happy experimenting     Rick  KL7CW        PS  I have never tried the MFJ whip
thanks Rick.. that railing runs the entire perimeter of the roof. Never thought of it before as a counterpoise.
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KL7CW

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Re: mobile 20m ground plane for roof?
« Reply #7 on: November 21, 2020, 06:08:39 PM »

just another thought, not necessarily a recommendation.  A screwdriver antenna with a long whip would be another possible option.  You could remotely tune an antenna to several bands.  If the total antenna length is well over 1/8 wavelength, the performance on 20 may be acceptable to you, and many of the higher bands should be fine also.  I have never owned one myself, since I usually build my own antennas, but some folks like them. I think the main point most of us would stress is to try and and make the total antenna length 1/4 wavelength, but for sure over or well over 1/8 wavelength where the performance begins to be "somewhat reasonable" whatever that is. This scheme will also work on 40, 30, and possibly 80 meters, but you may lose something like an S unit or even more.        Let us know what happens,               Rick  KL7CW
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K2JF

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Re: mobile 20m ground plane for roof?
« Reply #8 on: November 22, 2020, 04:45:31 AM »

Quick and easy; MFJ-1979 17' telescoping stainless steel whip and two radials.  You can also adjust the whip for other bands and roll up the radials according to your band of operation.

73,

Jeff
what do you think about it's ability to survive in high winds?
Quick and easy; MFJ-1979 17' telescoping stainless steel whip and two radials.  You can also adjust the whip for other bands and roll up the radials according to your band of operation.

73,

Jeff
If you want it to be simple and work, just get the 17 foot MFJ, and clamp it to the metal railing.  If the railing is of a reasonable size and the sections are welded or screwed together for good connectivity, then if the total length is say 16 or more feet in each direction, or even something longer in one or more directions, it will probably work just fine since you can adjust the whip length for whole system resonance.  The whip may end up 15, 16, 17, 18, 19 feet or something like that.  No problem except there may be some SWR and some common mode current on the coax.  Use a common mode choke and that solves one problem, and a tuner in the shack or rig(if it is even necessary) will solve the probably small SWR problem especially if your coax loss is only something like one or so dB.  Those of us who operate portable QRP typically have  fine results using a chain link fence, metal picnic table, etc. as our counterpoise.  Is this ideal? NO, but is simple and I really doubt that you would lose more than a dB or two. The advantage is it would be easy to tune and get you on the air immediately.  If the system is satisfactory, you can always replace the whip with something like a 10 foot piece of conduit with a fiberglass bicycle or snow marker on the top and use the whip for portable operation in a park if you do not think the whip will survive long term.  If you can install 3 or 4 elevated radials each 1/4 wavelength long this would naturally be a bit better.  Another advantage of the whip, when the upper bands open up in a very few years, simply shortening the whip for system resonance on say 17 or 15 meters would be a snap. Since we do not know if the railing is well grounded to the building, or has a resistive connection, it is impossible to predict what will happen.  However I have found that perhaps 95 % of the time when I operate portable, adding any large metal object either makes no change or improves performance.  Very seldom does this degrade performance....so with the limited info I have, lets say your odds of success should be good that will be a very useable antenna. If that antenna is a "keeper" you could keep that length and put a remote automatic tuner right at the base and use it on 20, 17, 15 and even possibly on 30 meters.  If you want a shorter antenna, make it resonant on 17 meters, and the remote atu should tune it up just fine on 20, 17, 15 and possibly other bands. If you will only use the antenna on the band it is resonant on, then a tuner in the shack (if necessary) should be OK with only a little additional loss.  If you want to use the antenna on a different band than it is adjusted for, then a tuner at the base of the vertical would be preferable.  Happy experimenting     Rick  KL7CW        PS  I have never tried the MFJ whip
thanks Rick.. that railing runs the entire perimeter of the roof. Never thought of it before as a counterpoise.
Hi Rick - I decided to get the two hamsticks and make a shortened dipole out of them for 20m and use the end fed antenna for the other bands. I ruled out the vertical for now but it is something I will get around to trying at some point. Once we head into spring 2021 there may be some work going on up on the roof and all antennas will have to come down.
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KH6AQ

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Re: mobile 20m ground plane for roof?
« Reply #9 on: November 22, 2020, 02:55:12 PM »

If the antenna is to be left up permanently the MFJ 17' telescoping whip is not an option. It cannot take much wind.

For the counterpoise, if you have the space, 2 or 4 radial wires would be best.
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KK4OBI

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Re: mobile 20m ground plane for roof?
« Reply #10 on: February 08, 2021, 02:23:47 PM »

I have used 17 foot MFJ telescoping antennas for years.  Generally as an MFJ vertical with one MFJ radial or as a 17 foot vertical with a 20 meter Hamstick radial.  The only problem that is over time, day-nite expansion and contraction cause the vertical telescoping sections to slip and raise the resonant frequency.  Hose clamps solve this problem.  Max winds experienced probably around 20 mph.
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KK4OBI - the Old Band Instructor
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KC9QBY

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Re: mobile 20m ground plane for roof?
« Reply #11 on: February 12, 2021, 05:41:39 PM »

Have a pair of screwdriver Wolf River Coils Silver Bullet Platinum (handles higher power) deck rail mounted.  6x32ft radials on each. Kind of center loaded, 102” solid stainless steel whip.   Shorter whip and/or alternate counterpoise definitely viable trial, whip particularly on 20M. Pix on my QRZ page.

Worthy aside: Shout-out KK4OBI awesome modeling work on bent dipoles. Required reading for attic installs, in my case.  Well done sir!
https://www.qsl.net/kk4obi/

73 and good luck,  Chuck  KC9QBY
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73,  Chuck  KC9QBY

KM1H

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Re: mobile 20m ground plane for roof?
« Reply #12 on: March 28, 2021, 01:08:53 PM »

Quote
Rick  KL7CW        PS  I have never tried the MFJ whip

Good info I think but you still havent mastered the concept of paragraphs so my eyeballs went into drift mode.

Carl
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