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Author Topic: Amount of radials for Verticals  (Read 11603 times)
KK4AXX
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« Reply #30 on: December 02, 2012, 11:14:28 PM »

Radials.... I sweated out having enough radials for my re-built 4BTV and before it my homebrew 17M copper-pipe vertical.

Eventually I decided that the best idea was to add what I had wire for at the time and more as I could.  My place is just west of Mobile, AL.  I have superb topsoil that grows anything I stick in the ground, but the underlying sand that makes for great drainage simply sucks for antennas.  Anyhow, I started with a dozen radials with the 17 Meter installation, but in the end after adding the 4BTV the number was 36.  I cut and stretched out 12 more, but when three of my regular contacts, (uostate NY, northern CA, and Aruba) couldn't tell a difference in my signal I pulled them off rather than take the time to step them into the soil like my others.  The first 24 were from phone cord and the last were some salvaged 14ga multi-strand.  Oh! I should add that there are (currently) four 8ft ground rods out there in the backyard and the antennas are tied into them and they are all tied into my common ground.  (With a little luck I'll finish ringing the house in 4ga before spring, and yes, all the connections are welded, including the service ground.)

All that to say this... Don't sweat the theory more than you enjoy the hobby.  Yes, you need radials.  Nope, no one can say with certainty that "X" number of "Y" length radials of "Z" gauge wire will make a top performing vertical.  Just go stretch what you can and try them out!

Geez... I'm getting yappy in my old age.
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George U. Potter Lodge, #912, F. & A. M. of Alabama
A.A.S.R., S.J. - Valley of Mobile, Orient of Alabama
Dave Langham Chapter, #536, Order of the Eastern Star
Order of DeMolay, Mobile Chapter, Adult Advisory Council
NO9E
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« Reply #31 on: December 03, 2012, 07:11:08 AM »

A wooden house with plastic siding is no problem. Brick or aluminum siding is a big problem. If soil conductivity is poor, the vertical's performance will be poor compared to a dipole regardless of the number of radials.

You may try an attic dipole, or an invisible dipole from thin wire (e.g., slinky 26 gauge) on top of your house fed by ladderline or TV flat line. Then compare with your vertical.

Ignacy, NO9E


...
  Unforutnately it might be that poor results are due to the proximity of the house, or it could just be other factors that are keeping you from making the contacts you want.  But with only eight radials of varying length in a small area, I would certainly try to improve the ground system.


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N3DT
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« Reply #32 on: December 03, 2012, 06:11:53 PM »

Believe me there's no reason to play with lossy ground radials. 

It's the ground that is lossy, not the radials. Radials reduce the ground losses.
But there's no reason even to have radials, radials are usually on or in the ground and therefore are lossy, ie. part of the ground.  Get rid of the radials and have an effective antenna with a real 50Ω input.  If you don't want to believe me, go for it.  I have low angle radiation, no ground issues and a multi-band antenna.  And I don't string wires all over the ground, just in the air.  All I do is isolate the antenna from the coax and ground with a ferrite current choke and all the problems go away.  It's really kind of like an off center fed dipole that's been choked off the ground.

All I can say is try it.  It's cheap, easy and only requires one support and some PVC for a spreader.  But there are Luddites all over the place, including me, but I don't put up with inefficient low radiation resistance verticals.   There's no need.  Things really do progress as time goes on.

Dave
N3DT
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RFRY
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« Reply #33 on: December 04, 2012, 02:31:00 AM »

Typically, a 1/4 wave ground mounted vertical over a very good, extensive radial system will have a feedpoint of around 36 ohms.....which will give an SWR of about 1.5 to one. This is the LOWEST SWR you will see if you feed the antenna directly with coax.

A suitable network located at the junction of the coax transmission line and the base of a 1/4-wave, series-fed monopole will transform the ~36 ohm +/-j X impedance to match the Zo of the coax. This will enable the monopole system to radiate close to 95% of the applied power, with a 1:1 SWR on the coax.

This is common practice for MW broadcast stations, who typically produce a radiated groundwave field at 1 km that is just several percent below that of a perfect monopole driven against a perfect ground plane, for that applied power.

Example:  For 1 kW of applied power, a perfect 1/4-wave monopole over a perfect ground plane generates a groundwave field of about 314 mV/m at 1 km.  A practical 1/4-wave monopole driven against 120 x 1/4-wave buried radials generates about 306 mV/m for those conditions.

The field change for the practical system is 0.975 X.  The corresponding change in radiated power is 0.9752 = 0.95 X.
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KE9PP
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« Reply #34 on: December 08, 2012, 09:40:35 AM »

Not actually an answer to the specific question about radials but if the original poster wants to get on the air, the response regarding "think mobile" reminded me of something.  While visiting there, I had great luck with a simple Hamstick on a mag mount placed on a 6' by 4' cast iron patio table in my son's back yard in Ohio.  I too was using 100 watts and worked the entire East Coast and the Midwest on 40 Meters during the daytime with just a Kenwood TS-130S and 100 watts.  I was using a Dentron Monitor Jr. matching device.  Just a thought
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KD4SBY
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Posts: 225




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« Reply #35 on: December 11, 2012, 05:54:02 AM »

Adjusting the antenna for minimum SWR is as good as any other method
of adjusting it, and better than some alternatives.  If the SWR is low then
the system must be close to resonance at that point, since you can't have
a low SWR in a 50 ohm system with high reactance.  And since the reactance
changes with frequency faster than the resistance, the point of minimum
SWR will be very close to resonance.

Adjusting the antenna for minimum SWR is easy and it works regardless
of the length of coax cable.  Don't believe those who try to tell you
otherwise.
I always was under the impression that the above was true, specially when measured at the antenna feedpoint itself, with an instrument intended  for a 50 ohm load. That is why I used that method to tune the antenna. In practice, I used a 100 ft cable and a tuner to keep my transmitter happy on all the frequencies used.
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W4VR
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« Reply #36 on: December 13, 2012, 08:33:42 AM »

Length of radials on the ground is not critical

Actually it does not even really matter. You can use any length you can fit in.

You are correct.  From my own experience I use a combination of 35 foot, 50 foot, and 100 radials on the ground for my 160 meter inverted L.  I now have a total of 50.  As I added radials, regardless of length, I could see the feedpoint Resistance drop significantly to the point where it would stabilize at about 40 radials. 
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G8HQP
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« Reply #37 on: December 13, 2012, 11:03:52 AM »

It seems to me that part of the confusion about ground radials is that people confuse them with counterpoises. A counterpoise (e.g. as seen as part of the VHF quarter-wave 'ground plane') has to be vaguely similar in length to a quarter-wave, and isolated at the far end so the near end presents a low impedance. It acts like a transmission line. Putting several counterpoises in parallel (electrically) but in different directions (spatially) reduces impedance still further.

A radial just at or above ground and isolated at the far end could act like a lossy counterpoise. Lossy because it is near ground. When the radials are much shorter than a quarter-wave and grounded at the far end then they instead act more like connections (multiple connections) to the ground. As the ground is not a good conductor a single connection will have a highish resistance, so we need lots of connections wired in parallel. This will work best when the connections are as spread out as possible. Two ground connections near each other will act like just one connection. So my conclusion (based on thought rather than experience) is that if radials have to be short (the usual amateur situation?) then have them different lengths so that the ground connections are as far spread out around the available area as is possible i.e. don't make them all as long as possible with the ends grounded at the boundary but have some shorter ones with grounds nearer the centre of the area.

Does that make sense?
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N6DMR
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Posts: 34




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« Reply #38 on: December 15, 2012, 08:28:03 PM »

I have a Butternut HF6V which is a 10m-80m vertical.  I travel full time in my RV and needed to be able to quickly set up and take down.  I built a wooden tilt base and mounted 16 each 25 foot metal tape measures around the 18" diameter base, tying them with a copper wire around the base.  I set them at 22 degrees apart.  I can pull the tapes out, lock them and bang, 16 each 25 foot radials.

Using this ground radial setup the Butternut on 80m had a very narrow bandwidth, about 25-20 kHZ which indicates a decent ground. With a poor ground, the bandwidth on 80m would be much wider which would indicate a poor or lossy ground plane (ie the dummy load theory).

With only 15 feet to run radials, I would run as many as you can reasonably (say 25-30) and you should do as well as possible.

I think the bigger issue is the poor antenna efficiency with a short vertical.  Phil Silas ( www.ad5x.com) has a number of very good articles on the 43 foot vertical where he compares this to a short (23 foot) antenna.  The smaller vertials are not too bad from 10m to 20m but on 40m and lower frequencies the antenna efficiency is lower than 50%.

ALL antennas are a compromise unless you have the space to put a 1/2 wave dipole up at least 1/2 wavelength.

As said earlier in this thread, you do the best you can technically and enjoy the result.

If you are looking to do something on say 20m, look into a Petlowany counterpoise similar to the taktenna concept.

http://www.n0lx.com/petlowany_ground.html


Duane
N6DMR
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RFRY
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Posts: 320


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« Reply #39 on: December 17, 2012, 08:13:15 AM »

According to the 1937 BL&E experiments and their IRE paper, the value of increasing the number of buried radials used with a monopole depends on their length.

If they are short (~0.15WL), then there is little improvement in radiated fields when going from 30 to 120 buried radials.   But that is not true when they are long (~0.4WL).

This is nicely summarized in the clip below from a 1947 IRE paper by Carl E. Smith, a well-known broadcast consulting engineer.

http://i62.photobucket.com/albums/h85/rfry-100/GndSystemLosses_zps0b36c41e.jpg

RF
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KA7NIQ
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« Reply #40 on: January 11, 2013, 07:41:22 AM »

Believe me there's no reason to play with lossy ground radials. 

It's the ground that is lossy, not the radials. Radials reduce the ground losses.
But there's no reason even to have radials, radials are usually on or in the ground and therefore are lossy, ie. part of the ground.  Get rid of the radials and have an effective antenna with a real 50Ω input.  If you don't want to believe me, go for it.  I have low angle radiation, no ground issues and a multi-band antenna.  And I don't string wires all over the ground, just in the air.  All I do is isolate the antenna from the coax and ground with a ferrite current choke and all the problems go away.  It's really kind of like an off center fed dipole that's been choked off the ground.

All I can say is try it.  It's cheap, easy and only requires one support and some PVC for a spreader.  But there are Luddites all over the place, including me, but I don't put up with inefficient low radiation resistance verticals.   There's no need.  Things really do progress as time goes on.

Dave
N3DT
Would you care to post more info, it sounds too good to be true.
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WB6BYU
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Posts: 13448




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« Reply #41 on: January 11, 2013, 12:52:51 PM »

Quote from: KA7NIQ

Would you care to post more info, it sounds too good to be true.



I think he is referring to the C-pole antenna. 

Like this:  http://www.qsl.net/hb9mtn/hb9mtn-c-pole.html
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N4CR
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Posts: 1688




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« Reply #42 on: January 11, 2013, 02:23:30 PM »

I think he is referring to the C-pole antenna. 

Like this:  http://www.qsl.net/hb9mtn/hb9mtn-c-pole.html

That's not an off center fed dipole.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
WB6BYU
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Posts: 13448




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« Reply #43 on: January 11, 2013, 02:28:33 PM »

It's basically a half wave wire (just like a dipole) with the ends bent around so
maximum current is in the continuous vertical section.  It is fed off-center
(at the bottom) where the impedance is 50 ohms (with proper adjustment of
the dimensions.)

It might not look like a conventional OCFD, but if you moved the feedpoint up
to the center of the continuous vertical section, you'd have trouble explaining
why it isn't a vertical dipole with bent ends.
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KA7NIQ
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Posts: 258


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« Reply #44 on: January 13, 2013, 10:13:41 AM »

It's basically a half wave wire (just like a dipole) with the ends bent around so
maximum current is in the continuous vertical section.  It is fed off-center
(at the bottom) where the impedance is 50 ohms (with proper adjustment of
the dimensions.)

It might not look like a conventional OCFD, but if you moved the feedpoint up
to the center of the continuous vertical section, you'd have trouble explaining
why it isn't a vertical dipole with bent ends.
So, IS it truly a vertical w/o any need for radials, or not ?
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