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eHam Forums => Elmers => Topic started by: KD4SBY on November 29, 2012, 06:33:25 AM



Title: Amount of radials for Verticals
Post by: KD4SBY on November 29, 2012, 06:33:25 AM
There have been many discussions on radials for ground mounted verticals, but I have an other question in that regard. Everybody knows that the more radials the better, but what are the penalties of having just a few, like eight of them of varying length? Living in a HOA I have very little real estate to begin with, and the rules have forced me to mount my 4BTV in the backyard 6ft away from my house. That leaves only half a circle of ground for the radials. Besides that, my backyard is only 15ft deep, not much for any length of radials. I have been operating with this antenna with mixed success, sometimes I do well, other times very poorly. (I can hear stations, they do not hear me with 100W - Ant. is tuned with analyser for low SWR) Can it be the amount of radials the reason? Comments anyone?


Title: RE: Amount of radials for Verticals
Post by: N3OX on November 29, 2012, 06:47:53 AM
I have been operating with this antenna with mixed success, sometimes I do well, other times very poorly. (I can hear stations, they do not hear me with 100W - Ant. is tuned with analyser for low SWR) Can it be the amount of radials the reason?

Could be.  You'll probably do better with a couple dozen than you will with 8.  Whether or not it's enough depends on a lot of factors, and whether or not it's your problem won't be clear until you try adding more.  Don't worry about how long they are, just go from the base of the vertical until you hit an edge of your area.  A probably good way to do radials in small irregular spaces is to mark out equal spacing of the tips around the edge and then run radials from the base to those points.  That way you end up with more of an even density. 

For a small area you could even consider making or buying a wire mesh.  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.



Title: RE: Amount of radials for Verticals
Post by: HB9PJT on November 29, 2012, 06:51:22 AM
For elevated radials 2 pcs. are enough. 2 pcs. per band. Or in total 2 pcs. but then you have to tune them for every band.

But in your case an antenna without the need of radials would probably be the better choice. I would go with a Hygain AV-620 or AV-640.

73, Peter - HB9PJT


Title: RE: Amount of radials for Verticals
Post by: K1WJ on November 29, 2012, 07:10:10 AM
You should be able to adjust antenna so no tuner is needed. Put as many radials as you can, based on data I've seen, 16 ground radials would be the minimum I would shoot far. Space them out the best you can 360. Length of radials on the ground is not critical, when I laid out my 32 ground radials on my 4BTV I used lengths of 8,16 & 33ft. Make sure to have coax choke close to antenna feed point. Bottom line: do what you can do & make the contacts you can, the band conditions will be the major factor here. I also modified the antenna for operation on 12m & 17m with 1/4w vertical wires - real easy to do - works good. 73 K1WJ David


Title: RE: Amount of radials for Verticals
Post by: W8JX on November 29, 2012, 07:41:40 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.


Title: RE: Amount of radials for Verticals
Post by: W5FYI on November 29, 2012, 07:56:32 AM
Here's what I did for a flagpole vertical in a small yard; I used insulated AWG18 wire, ran it straight out to the house, did a 90° or so bend, went out two feet or so, then straight back to the radial ring. Then back out to two feet from where its last bend was, did another 90° turn, then back to the ring, and so on until I had gone completely around the yard. Of course, at the radial ring I stripped the insulation and soldered each pass-through. When done, I sealed each solder junction with fingernail polish, then RTV silicone, and buried everything about an inch or so underground. In other words, the radial field is made of one continuous strand of #18 wire (I didn't cut the ends).

The ARRL Antenna Book lists a table that purports to give optimum radial numbers and lengths for anywhere from 16 radials up to 120. If you have the real estate to lay 120 radials at least 0.4 wavelength long, it will deliver the most power to the antenna. If you can stand a 3dB loss, then 16 radials 0.1 wavelengths long will suffice. It also says, "Using radials considerably longer...while not adverse to performance, does not yield significant improvement either," and "...the more total wire installed, the better the performance...."

Have fun.


Title: RE: Amount of radials for Verticals
Post by: K0BT on November 29, 2012, 08:20:35 AM
N6LF provided empirical information on this subject in a series of articles in QEX.  You can find the information at: http://www.antennasbyn6lf.com/design_of_radial_ground_systems/ under the heading "Series of QEX articles on ground system experiments".



Title: RE: Amount of radials for Verticals
Post by: ONAIR on November 29, 2012, 12:45:43 PM
Can one use buried aluminum foil or a section of chain link fence instead of regular radials?  Just wondering.


Title: RE: Amount of radials for Verticals
Post by: W8JX on November 29, 2012, 02:32:38 PM
Can one use buried aluminum foil or a section of chain link fence instead of regular radials?  Just wondering.

Chain link does not have a good bond through sections but field fence or chicken wire would work. Aluminum foil even if you could bond to it would not last long in soil if damp.


Title: RE: Amount of radials for Verticals
Post by: KK5J on November 29, 2012, 05:01:28 PM
You might look for LB Cebic's work on vertical performance with short radials. I think he published that in QST back in late 70's or early 80's if I remember correctly. He used a large number of short radials with his vertical with some success. I suspect it would probably be similar to a having a good wire mesh. Agree 100% with N3OX and others, you may have other performance factors you'll have to live with. Good luck.


Title: RE: Amount of radials for Verticals
Post by: VE3FMC on November 29, 2012, 05:32:07 PM
Here's what I did for a flagpole vertical in a small yard; I used insulated AWG18 wire, ran it straight out to the house, did a 90° or so bend, went out two feet or so, then straight back to the radial ring. Then back out to two feet from where its last bend was, did another 90° turn, then back to the ring, and so on until I had gone completely around the yard. Of course, at the radial ring I stripped the insulation and soldered each pass-through. When done, I sealed each solder junction with fingernail polish, then RTV silicone, and buried everything about an inch or so underground. In other words, the radial field is made of one continuous strand of #18 wire (I didn't cut the ends).

The ARRL Antenna Book lists a table that purports to give optimum radial numbers and lengths for anywhere from 16 radials up to 120. If you have the real estate to lay 120 radials at least 0.4 wavelength long, it will deliver the most power to the antenna. If you can stand a 3dB loss, then 16 radials 0.1 wavelengths long will suffice. It also says, "Using radials considerably longer...while not adverse to performance, does not yield significant improvement either," and "...the more total wire installed, the better the performance...."

Have fun.

I know a guy who has a Screwdriver set up like you have your vertical set up. One big loop of wire around the fence line in his back yard. He tells me that antenna works quite well, even on 160 with that setup. If I recall he told me he had about 250 feet of wire in his loop and it is just buried underneath the surface.


Title: RE: Amount of radials for Verticals
Post by: K2DC on November 30, 2012, 03:57:09 AM
, "Using radials considerably longer...while not adverse to performance, does not yield significant improvement either," and "...the more total wire installed, the better the performance...."

Have fun.

Brian Edward N2MF also published an extensive modeling study in QST sometime on the '70s that reached a similar set of conclusions:

-  The obviously biggest benefit comes from something vs. nothing.  Eight radials of 1/8 wavelength long was a large improvement over no radials.

-  There was some improvement over that from using 16 radials 1/4 wavelength long.

-  Beyond that, there is a diminishing rate of return from more or longer radials.  It certainly won't hurt, but you have to dedide if its worth the copper and the effort.

The bottom line is to do what you can and take what you get.

73,

Don, K2DC


Title: RE: Amount of radials for Verticals
Post by: KB3HG on November 30, 2012, 07:13:24 AM
Another way to look at a very limited space for radials. Think mobile HF. A few radials is like a small car. Many are like a big beast. All things being equal the beast will have better efficiency than the small car.  The ground plane is liken to a mirror, small mirror small reflection, a larger mirror a a bigger reflection.  Do what you can, a compromised antenna is better than no antenna. Do it and have fun.

Tom Kb3hg


Title: RE: Amount of radials for Verticals
Post by: W5LZ on November 30, 2012, 10:38:32 AM
Lets go back to that original post for a second.  'Tuned for the lowest/best SWR?', really?  Okay, but that's just half of tuning an antenna, and not the most important part anyway.  A dummy load has a fantastic SWR but makes for a terrible antenna with ot without radials...
 - 'Doc


Title: RE: Amount of radials for Verticals
Post by: N8YB on November 30, 2012, 01:28:10 PM
N6LF provided empirical information on this subject in a series of articles in QEX.  You can find the information at: http://www.antennasbyn6lf.com/design_of_radial_ground_systems/ under the heading "Series of QEX articles on ground system experiments".



This should be required reading for anyone considering installing a vertical antenna.


Title: RE: Amount of radials for Verticals
Post by: KH6AQ on November 30, 2012, 05:32:10 PM
Google N6LF and read the N6LF radial papers. For your antenna thirty 20' radials will do quite well.


Title: RE: Amount of radials for Verticals
Post by: VA3GUY on December 01, 2012, 10:47:00 AM
A while back, I had a Butternut HF6V vertical.  The Butternut sounded like the best option...especially because there are no traps.  I had it ground mounted in the back yard with only 13 radials of assorted lengths, mounted in a semi-circle (like a half moon...only 180 degrees) just under the surface of the ground and even though the bands haven't been great, generally, if I can hear them, I can work them.   In my opinion, it seems that the radials are not as important as indicated.  Bottom line though, you do what you can with what you've got!


Title: RE: Amount of radials for Verticals
Post by: N3DT on December 01, 2012, 07:28:01 PM
I'm going to repeat myself here.  I don't know why anyone puts up with ground radials.

Look up C-Pole antenna, get a  ferrite current choke for $20, make the antenna and forget the radials and ground issue, and have a good vertical antenna.

I have one cut for 80M, granted it's rather high, but it works on 160 through 10, but on some bands I also have one cut for 60M and it works on the bands the 80M one doesn't work on, like the higher ones, 17 and 10.

This antenna works great, hung from a tree, black wire, stealth, the wife doesn't see it.

But if you guys want to string all that wire out around your yard and still have a low impedance to feed, go for it.

Believe me there's no reason to play with lossy ground radials.  Verticals are not magic.

Dave
N3DT


Title: RE: Amount of radials for Verticals
Post by: KB4QAA on December 01, 2012, 09:21:52 PM
We like our lossy radials to go with our lossy traps and lossy coax.  ;)


Title: RE: Amount of radials for Verticals
Post by: K5LXP on December 02, 2012, 07:36:41 AM
Jerry Sevick, W2FMI did a study of short verticals and radials in the late '70's.  10 years ago he released a compilation of these works which you can get from the CQ bookstore.

http://store.cq-amateur-radio.com/Detail.bok?no=105 (http://store.cq-amateur-radio.com/Detail.bok?no=105)


Mark K5LXP
Albuquerque, NM


Title: RE: Amount of radials for Verticals
Post by: KD4SBY on December 02, 2012, 07:51:18 AM
Thanks fellows! From the feedback I get the impression that more radials will help, while the length of them is not that important. That is information I can use in my case. As I indicated, there is a limited amount of real estate for radials that I can install, but I did not mention that there are also two big trees in the yard with an extensive root system. That also make radials difficult to install, unless I make them shorter than I want. I will certainly follow up with that and the suggested reading of articles on this subject.

Lets go back to that original post for a second.  'Tuned for the lowest/best SWR?', really?  Okay, but that's just half of tuning an antenna, and not the most important part anyway.  A dummy load has a fantastic SWR but makes for a terrible antenna with without radials...
 - 'Doc

I included that note just to indicate that I did what I was suppose to do with the antenna itself, and there was little in that regard that I could do to improve things

I
A while back, I had a Butternut HF6V vertical.  The Butternut sounded like the best option...especially because there are no traps.  I had it ground mounted in the back yard with only 13 radials of assorted lengths, mounted in a semi-circle (like a half moon...only 180 degrees) just under the surface of the ground and even though the bands haven't been great, generally, if I can hear them, I can work them.   In my opinion, it seems that the radials are not as important as indicated.  Bottom line though, you do what you can with what you've got!

That sounds like my situation as far as radials is concerned! The  problem I have is that I often can hear them, but they can only hear me marginally. They do not get my call completely, for example, or my QTH. It can also be a matter of power, I use 100W, they may use 5 or 6 times that much. But you are right, I just might have to do with what I have!


Title: RE: Amount of radials for Verticals
Post by: NN4RH on December 02, 2012, 09:50:21 AM
A dummy load has a fantastic SWR but makes for a terrible antenna with ot without radials...


Wouldn't a dummy load make a terrible antenna even with radials?


Title: RE: Amount of radials for Verticals
Post by: NN4RH on December 02, 2012, 09:53:00 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.


Title: RE: Amount of radials for Verticals
Post by: NN4RH on December 02, 2012, 09:54:50 AM
Actually it does not even really matter. You can use any length you can fit in.

So radials one foot long would be as good as radials 30 feet long? Sorry. I don't buy that.


Title: RE: Amount of radials for Verticals
Post by: W8JX on December 02, 2012, 10:30:21 AM
Actually it does not even really matter. You can use any length you can fit in.

So radials one foot long would be as good as radials 30 feet long? Sorry. I don't buy that.

That is not what I meant. I was referring to fact that they do not need to be resonant when in ground contact. 


Title: RE: Amount of radials for Verticals
Post by: N4NYY on December 02, 2012, 11:40:52 AM
Actually it does not even really matter. You can use any length you can fit in.

So radials one foot long would be as good as radials 30 feet long? Sorry. I don't buy that.

Not what he is saying. To be on the safe side, I have heard radials as long as the center radiator. Most people do about 25-35ft lengths all the way around. Some do it longer. Some as big as the space will let them. No one does them 1 foot.


Title: RE: Amount of radials for Verticals
Post by: AC5UP on December 02, 2012, 12:34:47 PM
.........you would on a UHF ground plane antenna.
[ D'oh! ]       


Title: RE: Amount of radials for Verticals
Post by: WB6BYU on December 02, 2012, 01:28:00 PM
Quote from: KD4SBY

Quote from: W5LZ
Lets go back to that original post for a second.  'Tuned for the lowest/best SWR?', really?  Okay, but that's just half of tuning an antenna, and not the most important part anyway...

I included that note just to indicate that I did what I was suppose to do with the antenna itself, and there was little in that regard that I could do to improve things



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.

You'll still find a few misled diehards who insist that you tune it for zero reactance
rather than minimum SWR.  Let's see where that leads...

Let's say we have a 20m vertical with a 5uH loading coil inserted 6' above
the feedpoint, and we are adjusting the length of the upper section above
the coil for resonance at 14.1 MHz.  My EZNEC model suggests a total
length of 11.21 feet, with a input impedance of about 30 ohms or so
depending on the ground loss.  That's tuning the antenna for zero
reactance at the desired frequency:  where is the minimum SWR?  At
14.1225 MHz, if you have a very precise meter.  But the SWR curve is
very flat:  the SWR at resonance is 1.67 : 1, and it is less than 1.7 : 1
from 14.06 to 14.190 MHz.  A shift of 22kHz just isn't going to make
any practical difference in the real world.

But who wants to lay flat on the ground to take measurements right
at the feedpoint?  That's not very comfortable.  So let's say you add
a short coax jumper - say 5' long - between the feedpoint and the
antenna analyzer or SWR meter.  If you tune the antenna for minimum
SWR you get the same reading.  But if you are looking at the reactance
on an SWR analyzer it now looks like +20.7 ohms, even though the SWR
hasn't changed.  Now if you adjust the antenna length or X = 0 you end
up with an antenna length of 10.925 feet, a difference of over 4".  Worse
yet, the SWR at that point is 2.68 : 1.  Why would you want to tune
the antenna to that setting?

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.


Title: RE: Amount of radials for Verticals
Post by: N4NYY on December 02, 2012, 07:37:22 PM
.........you would on a UHF ground plane antenna.
[ D'oh! ]       


You are not groundmounting a VHF or UHF antenna.


Title: RE: Amount of radials for Verticals
Post by: K0ZN on December 02, 2012, 07:53:04 PM
If you have a "low" ( i.e.  near 1:1 SWR) it is because you are losing about 50% of your transmitter output to Ground Losses.  A low SWR shows that
ohmic resistance on the lossy earth ground is ADDING to the antenna radiation resistance and bring up the TOTAL resistance to near 50 ohms, thus
showing a good match for the coax.

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.  Low SWR is NOT always a reliable or accurate
indicator of antenna EFFICIENCY.  To wit:  A typical center fed Zepp will usually have a very high SWR, but the antenna SYSTEM is very efficient if fed
with open wire line or ladderline.

SWR is nothing more than electrical condition that exists on the transmission line.  It can be harmless or bad depending upon the TYPE of transmission line
and the coupling system ("Antenna Tuner") between the transmitter and the line.

In-ground radials do nothing more than reduce ground losses as previous posts note. They are NOT resonant. Simply put: plain old dirt SUCKS as a conductor of RF energy....copper wire conducts very well.....so obviously, you want MOST of your RF power flowing in copper wire and not plain old dirt !

Rough statement:  if you put in about 50 to 60 radials, regardless of length (assuming reasonable length) you will be pleasantly surprised at how well your
antenna will work.

Most of the hams that are unhappy with basic vertical performance don't put in an adequate ground system.  It is just Ohm's Law!!

73,  K0ZN


Title: RE: Amount of radials for Verticals
Post by: KK4AXX 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.


Title: RE: Amount of radials for Verticals
Post by: NO9E 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.




Title: RE: Amount of radials for Verticals
Post by: N3DT 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


Title: RE: Amount of radials for Verticals
Post by: RFRY 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.


Title: RE: Amount of radials for Verticals
Post by: KE9PP 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


Title: RE: Amount of radials for Verticals
Post by: KD4SBY 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.


Title: RE: Amount of radials for Verticals
Post by: W4VR 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. 


Title: RE: Amount of radials for Verticals
Post by: G8HQP 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?


Title: RE: Amount of radials for Verticals
Post by: N6DMR 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


Title: RE: Amount of radials for Verticals
Post by: RFRY 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


Title: RE: Amount of radials for Verticals
Post by: KA7NIQ 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.


Title: RE: Amount of radials for Verticals
Post by: WB6BYU 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


Title: RE: Amount of radials for Verticals
Post by: N4CR 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.


Title: RE: Amount of radials for Verticals
Post by: WB6BYU 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.


Title: RE: Amount of radials for Verticals
Post by: KA7NIQ 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 ?


Title: RE: Amount of radials for Verticals
Post by: WB6BYU on January 13, 2013, 10:56:49 AM
It does have primarily vertical radiation, and it doesn't use ground radials.

I modeled it in EZNEC and compared it against a ground plane with the same
maximum height using 4 elevated radial wires about 4" above ground.  The GP
had a bit more gain for identical ground conditions - in the range of 0.6dB.
I didn't try putting a set of radials under the C antenna, using more radials on
the ground plane, varying the height of either antenna, different ground
conditions, etc.  But they give essentially equivalent results, and minor
changes to one or the other might shift the advantage.

The model still estimates about 5dB of ground losses for both antennas.

It can be used on multiple bands with a tuner, though the SWR will be high
(the model suggests >100 : 1 on all bands except 20m and 10m, and the
pattern isn't exactly optimum on 10m.)  I'd consider it a single band antenna
unless you have the tuner right at the feedpoint.  Common mode currents
are likely to be a problem:  one choke balun might not be enough, especially
on other bands besides 20m.

It probably is most similar to a vertical dipole with the ends bent around, but
a ground plane with elevated radials is basically a dipole anyway.

The dimensions aren't nearly as critical as some of the online calculators might
make it appear:  I've modeled versions with narrow or wide spacing with little
change in overall performance.  Wide spacing requires less overall height, and
affects the relative lengths of the wires on the open side for a 50 ohm match,
but you can customize the dimensions to meet your specific needs.


If you were to straighten out the top wire then you could get it to work as
a vertical dipole with one end folded on 20m, and a J-pole on 10m.  (I have
a design for 80m / 40m that I want to try some time using that approach.)
That would require more height, of course.


Title: RE: Amount of radials for Verticals
Post by: N4CR on January 14, 2013, 05:33:29 PM
So, IS it truly a vertical w/o any need for radials, or not ?

It's vertically polarized on one frequency. It doesn't need radials. It's a single band antenna.

A typical OCF dipole works on multiple frequencies but that one depends on the current maximum to be in the center of the vertical section of wire which only happens on and around a single frequency. (and to some extent, it's harmonics)