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Author Topic: 40m Vertical - Elevated radials question  (Read 3820 times)
EI2HPB
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« on: August 08, 2012, 06:56:50 AM »

Having just recently aquired my licence I am starting to put my HF station together. I have a very small garden like most people here in Ireland so antennas are always a challenge. Fortunately I do have a small tower so I can get a smaller yagi up for 20-10m but 40m and 80m will be somewhat of a challenge. I have decided to forgo 80 for now and try to get a decent signal out on 40. I have been looking at various antenna ideas and I think a vertical might suit my situation best. Due to the tiny lawn area a ground mounted antenna is not possible because I have no room for radials, and also have young kids playing out there...
I was looking at a monoband 40m vertical which obviously requires radials but the only possible location for such an antenna would be mounted on a pole in the corner of my garden. My questions are, what height should the base of the antenna ideally be from the ground or does it matter? I believe as little as 2 radials should work in an elevated install, is this true and what lenght is optimal? I have got a wooden fence about 5ft high and about 30ft long running both west and south from the corner where I'm thinking of installing the antenna and I thought I might tack the radials to the top of that? Otherwise I am open to any other suggestions for a decent 40m antenna in a very small space.
EI2HPB
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W5DXP
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« Reply #1 on: August 08, 2012, 07:44:00 AM »

Why not gamma-feed your tower for 40m operation?
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W8JX
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« Reply #2 on: August 08, 2012, 07:45:47 AM »

When you elevate it the radials have to be tuned to frequency in use. When ground mounted, they do not have too. I would suggest you give serious thought to simple ground mounting the vertical and attaching radials of whatever length you can fit in as when they are in contact with ground they do not need to be resonant. Myself I have found that even ground mount a 40m vert can do very well. In the scheme of things houses and trees have little impact of a 40m radio wave.
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K1WJ
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« Reply #3 on: August 08, 2012, 08:08:55 AM »

You could go with the base at 5ft as you described, with a single 1/4 wave 40m radial also held up 5ft high on the insulator side by a camera tripod or something similar. I have had very good success with an elevated 1/4wave vertical with only a single elevated radial. If you could make it high enough ( like 7-8ft ) at the base, kids will not be able to reach it, plus being higher is a plus. 73 K1WJ David  Cool
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EI2HPB
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« Reply #4 on: August 08, 2012, 08:09:49 AM »

Why not gamma-feed your tower for 40m operation?

Sound interesting... how is it done?

I would suggest you give serious thought to simple ground mounting the vertical and attaching radials of whatever length you can fit in as when they are in contact with ground they do not need to be resonant. Myself I have found that even ground mount a 40m vert can do very well. In the scheme of things houses and trees have little impact of a 40m radio wave.

I really have very little grass in my yard, not enough for any decent radial field, and I have young kids so transmitting while they are out mooching around in the yard would be a worry... in case they were holding the antenna or leaning against it... or trying to climb it.. loll
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EI2HPB
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« Reply #5 on: August 08, 2012, 08:15:26 AM »

You could go with the base at 5ft as you described, with a single 1/4 wave 40m radial also held up 5ft high on the insulator side by a camera tripod or something similar. I have had very good success with an elevated 1/4wave vertical with only a single elevated radial. If you could make it high enough ( like 7-8ft ) at the base, kids will not be able to reach it, plus being higher is a plus. 73 K1WJ David  Cool

Sounds better... the 40m monobander I'm looking at is about 33ft tall so I dont want to go too high with my mounting pole, but 7-8ft is no problem... I could actually get two elevated radials on the 5ft wooden fence, one running west and the other south... would this make the antenna slightly directional?
« Last Edit: August 08, 2012, 08:42:53 AM by EI2HPB » Logged
K1WJ
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« Reply #6 on: August 08, 2012, 09:05:36 AM »

If you can do 2 elevated 1/4 wave 40m radials - do it.

It will give it a more uniform pattern, & may favor in direction or sides in which elevated radials are present.
With 2 elevated - you will be good to go. All set.

With mine, I set my vertical element length, 1st establish that - I did 20m so my vertical was about 16.5+-ft in length.
Then I made the radial long about 17ft - then started checking swr, cut off 1/2 inch of radial per time to get swr to 1.5/1.
Took about 4-5 trims to get there. All set. Hundreds of contacts. 73 K1WJ David Cool
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W5DXP
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« Reply #7 on: August 08, 2012, 11:04:42 AM »

Sound interesting... how is it done?

A google search will yield lots of information for "antenna gamma match". Let's assume your tower is 1/4WL (~10m) and resonant on 40m. The feedpoint impedance at the grounded base is zero ohms. The feedpoint at the top of the tower is a couple of thousand ohms. Between the base and the top of the tower, the feedpoint impedance is around 50 ohms at a certain point. If we install an inductive stub with a series capacitor to neutralize the stub's inductive reactance, we can transfer that 50 ohm impedance to the base of the antenna (or to the center of a plumber's delight dipole)..

Your tower is probably not resonant on 40m but it may be in the ballpark. A gamma feed with a matching network at the feedpoint will probably yield a 50 ohm impedance for the coax connection. I've even seen the beam on the tower used as a top-hat capacitance while the tower is being used as a monopole vertical.

So, how tall is your tower?

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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K3VAT
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« Reply #8 on: August 09, 2012, 02:34:39 AM »

EI2HPB:  Obviously you're looking at a compromise antenna and you have valid safety concerns.  The elevated vertical with tuned radials described by K1WJ may be you're best option.  But there some very important caveats to understand.

If you can do 2 elevated 1/4 wave 40m radials - do it.  It will give it a more uniform pattern, & may favor in direction or sides in which elevated radials are present.  With 2 elevated - you will be good to go. All set. ... 73 K1WJ David Cool 

An elevated verticals with 2 elevated, tuned radials can be made to radiate (and thus make contacts), but there are several important factors that one need consider.  There is no magic in the elevated radial system despite some claims.  They are very hard to properly tune and just obtaining a good SWR is no guarantee that your radials aren't radiating like hell and that the antenna pattern is significantly skewed.  Recent research by Rudy Severns, N6LF has validated earlier studies by Dick Weber, K5IU and Arch Doty, W7ACD showing how difficult it is to obtain proper symmetric current when using elevated radials.  For the technical discussion please see:  http://rudys.typepad.com/files/december-2010-letter-to-qst-technical-correspondence.pdf and Rudy's series on Elevated Radials.

Additionally, Tom, W8JI explored the common-mode current isolation problem inherit in verticals with elevated radials and his research is found here: http://www.w8ji.com/verticals_and_baluns.htm
I have not seen any info that would support the hypothesis that with two elevated radials that the resulting radiation pattern will favor the sides where the radials are present.

EI2HPB:  Hopefully, from the above references one can see that important factors (such as a properly functioning balun) need to be considered and you might want to consider adding another pair of elevated, tuned radials or as W8JX suggested earlier figure out how best to install a ground-mounted vertical system (with safety for the children).

GL, 73, Rich, K3VAT

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EI2HPB
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« Reply #9 on: August 09, 2012, 06:55:38 AM »

Thanks for all the info... after everything I've read so far its seems like a fair amount of trouble putting up an elevated 40m vertical for what is essentially still a compromise antenna... I spent quite a while last evening looking at my yard again and taking measurements and for now I think will just hang a 40m dipole from the tower and tie it off on the fence on one side and figure out somewhere to tie it to on the back of the house... my missus probably wont be too pleased but she'll get over it... Cheesy ... If I ever move house again the next place will have a dedicated patch of lawn for ground mounting antennas!
EI2HPB
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W8JI
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« Reply #10 on: August 09, 2012, 07:58:24 AM »

Thanks for all the info... after everything I've read so far its seems like a fair amount of trouble putting up an elevated 40m vertical for what is essentially still a compromise antenna... I spent quite a while last evening looking at my yard again and taking measurements and for now I think will just hang a 40m dipole from the tower and tie it off on the fence on one side and figure out somewhere to tie it to on the back of the house... my missus probably wont be too pleased but she'll get over it... Cheesy ... If I ever move house again the next place will have a dedicated patch of lawn for ground mounting antennas!
EI2HPB

Ground systems are big complicated mess, and it is easy to reach conclusions in one situation that are not valid in others. A short distance away from Rudy N6LF, another person obtained different results and had to use much more radial height for similar results. Making things much worse, most articles never made a single reliable comparison field strength test, yet concluded measurements that could not determine efficiency were somehow measuring efficiency!!!

Resonance of elevated radials is one example of many opinions. An antenna system won't work significantly better with radials resonant as with them off tune and the element reactance compensated, except for common mode current induced on the feeder. In the end, unless you do something grossly wrong, everything is roughly the same except feedline common mode shield current issues.

If you just want to get on the air, your dipole is a wise choice. It will work as well as, or better than, a less-than-ideal vertical on 40M. Unless you do something wrong, a low dipole will let you work all over the world on 40 meters.

73 Tom
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WA8FOZ
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« Reply #11 on: August 09, 2012, 08:40:47 PM »

Quote
If you just want to get on the air, your dipole is a wise choice. It will work as well as, or better than, a less-than-ideal vertical on 40M. Unless you do something wrong, a low dipole will let you work all over the world on 40 meters.

What he said. You can have a lot of fun with such an antenna, and decide later if you want something more elaborate. FWIW, if you get interested in 75-80 meters, you could make a dipole of similar size with loading coils. Something like this:
http://www.iw5edi.com/ham-radio/?a-short-dipole-for-80-meters,31

Not a rock-crusher, but it would get you on. Here's a two-bander:
http://www.youtube.com/watch?v=wgIbWafVnSc

More than you need to do to get started, but an interesting project.

Welcome to HF!

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KC4MOP
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« Reply #12 on: August 10, 2012, 04:21:51 AM »

I have a 33 foot, insulated from ground, vertical. A lot of radials and it resonates at 40M no sweat. And a bonus!! 15M too.
I supported a separate wire held away from the main mast by PVC pipe and a separate tuning network gives me a 5/8 wave 17M antenna. I used the MFJ antenna switcher to let me select the two antennas and usually a series coil in the feed will get the 5/8 wave tuned for 50 ohms. There is a lot of good info on 5/8 wave antennas. Lower angle radiation and even some gain.

Fred
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W8IFI
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« Reply #13 on: August 10, 2012, 04:53:21 AM »

Some pretty good advice in the comments here. I agree a dipole might be the best solution. I had a ground mounted vertical with only about 4 radials  and couldn't get anything. I raised it so the radials were just over head high and got pretty fair results. sometimes that helps, sometimes doesn't. So many variables. Depends a lot on soil, moisture conditions, etc. Most formulas are based on perfect ground which very few of us have. So a little trial and error experimenting would be useful.
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