Isolating vertical endfed antennas from their support structures

[loosecannon]

hi all,

im reading alot about this lately here in the forum, and i was wondering if anyone could give me the low-down on why this is done?

what does it accomplish?
why is it necessary?
when is it not necessary?
what type of antenna(s) does this benefit?
how does this practice relate to lightning protection?
etc...

thanks for any and all info,
LC

 

[BOOTY MONSTER]

it looks like the way to go on a 2000 or a 99 to me . heres a thread that says it far far better than me .

http://www.worldwidedx.com/cb-antennas/21937-imax-2000-ground-plain.html
post # 7 thanks freecell

"every antenna is a dipole, if it isn't it doesn't radiate, not very efficiently anyway. balanced currents in both "poles" and elimination of common mode current on the feedline and the support structure is a must in any properly designed antenna system. while i don't voice it as often as i once did, low feedpoint heights, feedline and support structure radiation are the main culprits in most tvi and interference situations. in the case of my own antenna systems i make extensive use of baluns, radials and antenna tuners, even with such antennas as the "glass sticks" like the A99 and others and i really don't worry about rf earth-ground returns for antennas that are properly balanced and decoupled. come to think of it, swr is usually the last thing i worry about after poles, (two halves of the antenna) feedline and support current measurements and making sure that the transmitter is always looking into what it takes to keep it happy.

it's not too difficult to understand why most operators fail to realize any benefit from elevated radial systems (or add-on radial kits) when you stop for a moment and ask yourself if there is any antenna current present in the radial system to begin with. if the current has disappeared into the feed and support then what's left for the radials? unfortunately in the majority of cases, there's a lot more involved to proper antenna design and installation than just throwing up the latest entry on the market, adjusting for min. swr and flipping the switch and you and others here know that.

there are two things that can be done to insure the presence of current-as-charge in the added radial system for it to function properly. installing a current choke or a 1:1 current balun will create a high impedance reducing or eliminating common-mode currents on the outer shield of the feedline. the second thing that can be done is to insulate the antenna from the mast or support structure.

once these steps are accomplished the charge-as-current has no choice but to fill the radials with charge when they are added. the ultimate goal is to produce half of the total current generated at the feedpoint in the main radiator and to have the other half divided up evenly among the total number of radial elements.

ideally ALL common-mode current must flow in the radial system for it to function properly."

makes sense to me ....

 

[Marconi]

I don't understand all there is about this issue maybe, but I think the idea is one that has to do with a method for improving the decoupling of the antennna feed point from the earth thus allowing the antenna return currents to work better between the radiator and the GP elements and thus improving Field Strength.

I believe that some method of RF choking is also necessary in order to keep these currents off of the feed line as well. I think this might apply to any vertical antenna.

I tried it with my A99 just recently and I think my results proved to me that there is something to this idea, but conditions are terrible right now with DX trying to roll and all the local signals in the Houston area are variable and unpredictable at best. As I said, I got marginal results, but I did not fully insulate my antenna either. The insulating idea that seems to be popular with the end fed types is to add insulation to the end of the mast and then raise the antenna mount above the grounded mast. I simply installed PVC into my mount and left the antenna mounted beside the mast as it is typically done.

Lookup some threads posted by Multimode200 on this subject. He had remarkable improved results in the range of 3-5 sunits when insulated his SilverRod or something like that. It is a 1/2 wave like the A99 I think. Do a search for his name or try the word insulate or insulated.

I have an opinion about this, but you check it out first and maybe I will get into this idea some more with you after considering MM200's ideas and work.

I just noticed BM's posted the words of freecell describing this issue very well some time ago. Look that thread over also.

 

[RickC.]

The ones I've built (both voltage fed and j-poles) have had wood and fiberglass for supporting structures and did have not shown any problems so there may be something to this.

As an experiment I did use a short counterpoise for a while with the voltage fed half wave, didn't see any difference. The latest j-pole is still cooking along, and will until I have the roof redone later this month.


Rick

 

[jazzsinger]

i totally agree with freecell on this one for best performane the radiator needs something to push against,which in this case is current flowing on the radials equal to the current flow on the radiator. one problem people run into in their rush to do this is they insulate the mast,choke the cable then totally defeat the purpose by adding a ground wire to the antennas u bolts above the choke/insulated mast.by doing that you instantly replace the mast with the ground wire as a radiating element.not good as you've just defeated everything you set out to do.

the way round this is to add the ground wire preferably at ground level direct from the coax shield and keep it short,which will dissipate static and give lightning a low resistance path too ground (not that it will make much difference mind unless you unplug the coaxial cable and put it outside the house during storms,seeing as lightning will most likely still supply enough current via the centre conductor and shield once the current divides up and still do major damage to any radio equipment still connected not to mention anything else in its path to ground via the mains circuit).lightning does not f@ck around,underestimating its power can be fatal.it doesn't have to be a direct antenna strike either as even a lightning strike in close proximity can provide enough charge to damage radio equipment as has been seen here in the uk recently on 2 occasions i know of.

the other place you can fit the ground wire is below the rf choke/current balun/line isolator,which although a bit long (ideally ground wires should be less than a 1/4 wave off the frequency being used) and may induce some rf on it,it is still better than no ground at all,as it will give static a route to ground,which with many radios now using mosfet final transistors that are static sensitive is a very wise precaution.

 

[W5LZ]

Just something to think about.

Why do 'fan' dipoles, multiple dipoles fed by the same feed line at the same feed point, work? Basically because RF seeks the path of least resistance/impedance, right? If RF sees a part of that combination of multiple antenna parts it likes better than the others it is going to take it rather than a part of that antenna that it doesn't like. So, those 'other' parts for other bands might as well not be there as far as that RF is concerned.
So what does that have to do with what we're talking about here? What do you have if you stand that fan-dipole up on one end, instead of laying it out horizontally? You've basically got a multi-band vertical dipole, which is so close to a groundplane or end fed 1/4W, 1/2W, 5/8w, 'ad nausium', that there's no practical electrical difference.
And the 'tip-off', the part that can make a really big difference, is to -not- make that mast, ground wire, whatever, some length that the RF 'likes' as much as, or better than, a radiating part of the antenna. (And, yes, those radials, or car bodies, or ground radials, or just plain dirt, DO radiate!)
Makes sense to me, does it to you?

RF grounds and 'Safety' grounds...
... are two completely different animals! They may have a few things in common, but they also have a lot of differences, reasons for being. One is very seldom any benefit if used as the other, and probably won't be very compatible without some work/thought. The easiest thing to say is don't confuse the two.
- 'Doc

 

[RickC.]

. Directly connect it to an aluminium mounting pole and cut the mounting pole accordingly until you get a half decent match. Obviously you wont get a 75 OHM match because of the physical differences\inductance between the antenna and the pole but it would be interesting to see how close you could get.

Probably not very close given the impedance at resonance of an end fed half wave is on the order of 4-5K, and that's what I saw as best as my noise bridge would measure. I did the little trick of adjusting the L/C network with a 4500 ohm resistor for minimum VSWR, and the match on the actual antenna was near perfect, so that would seem to bear that out.

I assume you mean having the half wave portion itself insulated from the mast.

 

[RickC.]

The impedance of the mounting pole cut at a certain length may or may not match that. Imagine horizontally mounting the job lot so that the antenna is one half of a dipole and the mounting pole is the other. It would be no where resonant at 11m I know but I just wondered if you could get a match. Maybe that would cut out the need for as many winds on a matching coil and therefore reduce losses.

I think we're talking around each other on this one. You had mentioned an end-fed half wave connected directly (to coax, I assume) and I'm guessing the braid to the grounded pole. No matter where you cut that pole, I don't think you're going to find a good match to 50 ohms at the frequency the half wave is cut for. Some interesting things do happen with off-center feed on horizontal antennas, but this is a different thing you're talking about.

That half wave is being fed at the voltage peak and thus high impedance, no way that will be a good match to 50 ohms. Looks to me as though the feedpoint will need to be moved close to a voltage null to find an impedance close enough to match, but then it's no longer a half wave.

What you're describiing sounds to me like just changing the length of the "counterpoise" and my thought is that the feedpoint impedance would not change drastically. Just for grins when I first put up my end fed half wave which was voltage fed I tried it with a counterpoise, without one, and connected to ground. At that time I already had the noise bridge out of the circuit and was not looking for changes in impedance, but in RSSI and noise. The differences in those were not noticeable, but there was a slight elevation in VSWR, so the impedance likely did change some. The feedpoint was just over 1/4 wave above ground, the effect may have been different if were 1 1/2 wavelengths up as many CB antennas are. But this was with the matching circuit in place, not with it removed as you're talking about.

This would be easy enough for someone to try if they had the time and could measure impedance, but I'd be surprised if the feedpoint even approached 500 ohms, much less 50.




Rick

 

[jazzsinger]

The impedance of the mounting pole cut at a certain length may or may not match that. Imagine horizontally mounting the job lot so that the antenna is one half of a dipole and the mounting pole is the other. It would be no where resonant at 11m I know but I just wondered if you could get a match. Maybe that would cut out the need for as many winds on a matching coil and therefore reduce losses.

what your suggesting is basically a fullwave dipole,would have a very high feedpoint impedance and would be nigh on impossible to feed with coax.

 

[loosecannon]

thank you all for your replies.

this thread has been very informative.

i think this will be the next big thing for CB'ers to try when they get their new antennas.

i will certainly be mounting my alpha 5/8 like this when the time comes.

i'll probably ground the coax to a ground rod for some static dissipation also.

everything said here makes good sense to me, especially the part about masts and coax radiating and causing TVI.

very cool info,
LC

 

[Marconi]

hi all,

im reading alot about this lately here in the forum, and i was wondering if anyone could give me the low-down on why this is done?

what does it accomplish?
why is it necessary?
when is it not necessary?
what type of antenna(s) does this benefit?
how does this practice relate to lightning protection?
etc...

thanks for any and all info,
LC

LC, you are right this is a good thread, but in your final considerations for what RF does, how it acts, and where it goes, don't forget the sage advice by our friend 'Doc, which I have re-posted below:

Just something to think about.

Why do 'fan' dipoles, multiple dipoles fed by the same feed line at the same feed point, work? Basically because RF seeks the path of least resistance/impedance, right? If RF sees a part of that combination of multiple antenna parts it likes better than the others it is going to take it rather than a part of that antenna that it doesn't like. So, those 'other' parts for other bands might as well not be there as far as that RF is concerned.
So what does that have to do with what we're talking about here? What do you have if you stand that fan-dipole up on one end, instead of laying it out horizontally? You've basically got a multi-band vertical dipole, which is so close to a groundplane or end fed 1/4W, 1/2W, 5/8w, 'ad nausium', that there's no practical electrical difference.
And the 'tip-off', the part that can make a really big difference, is to -not- make that mast, ground wire, whatever, some length that the RF 'likes' as much as, or better than, a radiating part of the antenna. (And, yes, those radials, or car bodies, or ground radials, or just plain dirt, DO radiate!)
Makes sense to me, does it to you?

RF grounds and 'Safety' grounds...
... are two completely different animals! They may have a few things in common, but they also have a lot of differences, reasons for being. One is very seldom any benefit if used as the other, and probably won't be very compatible without some work/thought. The easiest thing to say is don't confuse the two.
- 'Doc

and remember, just because we do something thoughtful that is suggested by another does not always relate to success in our work or efforts without a true test of those results. Just because we have a choke added to our antenna does not insure that we have solved a problem that we are not really sure we have where the magnitude is great enough to really matter. So, often times we may do a lot of extra work for naught and for sure if we do not test and make sure as in before and after testing. Are you prepared to do that which is required or is just doing something that looks good---really good enough. Like ‘Doc suggests on another topic: …are we destined to always confuse the two?

The following is not to imply that I don't make mistakes and make some bad judgments, but in my recent testing, tuning, and comparing of several CB type vertical antennas I have here, I noticed a trend that I now consider very important when seeking maximum performance with these particular tunable type antennas we talk about and use.

After doing a number of iterations in this process it dawned on me that it might be likely that most of the tunable type antennas we use probably have some efficiency limits to the actual tuning beyond the specific design frequency limits imposed by such a design. For me, this means in simple words, that not all antennas are really tunable in the purest sense of the word and without degradation---even though they have physical tuning capabilities.

Below I will comment about the A99, but here I must talk about the I-10K for a moment. With the following ideas in mind, my experience with the I-10K is unique among all the antennas I tested this go-around. Both myself and Bob85 have commented that in our tuning efforts with this antenna we notice the match and resonance are never at the same frequency and we then wonder why and what affect that has on performance. This is just to present the idea as a problem for which neither I or Bob85 have a good answer or solution to the tune. If any one would like my notes, and BW reports, demonstrating this condition, they are available on request plus there are old archived threads regarding such discussions.

Example is the A99, which is likely designed both in physical length and with the matching to be a mono frequency device (probably the middle of the CB band) with capabilities due to a nice broad bandwidth to go up or down frequency a certain range with probably more minimal compromises to efficiency in doing so that tuning to your working frequency within the CB band. With this idea in mind to address the possible affects of the magnitude of common mode currents (the subject of this thread) on the antennas we all use. What I think I noticed here was, if I tune the antenna as best I can directly at the feed point for 27.205 I get a very nice smooth broad bowl shaped bandwidth curve, but when I try to tune at 27.385 I might get a nice tune for the radio, but the bandwidth curve becomes irregular in shape. This implies that the length and the matching coil are no longer as symmetrical as was the case at 27.205 and that even though I have a good control over the resistance in the match, I suddenly see a rapid increase in the changes in reactance; which our friend ‘Doc as said from the beginning, “…reactance don’t radiate, period.” My conclusion is that if this is close to reality, then with asymmetry showing its ugly head and reactance on the rise---is it any wonder that we may be seeing degradation in performance and for me isolation and a choke will not fix that problem. So, I say tune your mono-frequency CB antennas to the design frequency and use that nice bandwidth the antenna can provide to go in frequency where you want to.

Believe me a little increase in SWR operations is not at all bad even if you do run a big old hooten-nater, if it’s any good compared to what you will get operating with a lot of reactance present in the system even if you show a 1:1.1 SWR.

Now don't ya'll get scared and not respond, I don't bite "...I aint got no teeth."

Old Grandpa Corpus Christi, Texas, live and in color on your two way radio.

 

[W5LZ]

No, it doesn't mean a 1/2 wave dipole won't have a 75 ohm or less input impedance. Most of them certainly do, IF they are fed in the center. One of the things determining an antenna's input impedance is -where- you feed them.
- 'Doc

 

[jazzsinger]

Yes it is a full wave dipole. Are you saying that you cannot possibly get a low impedance on this imaginary antenna? So that would mean if I used this antenna for the 20m band as a half wave dipole I couldn't get a decent SWR and 75 OHM impedance?

no it would give a reasonably high swr on 20m,but would be most likely be within a couple of hundred ohms and matchable to coax with the right transformer.

on 22m/13.6mhz where it would be a resonant halfwave it would give a decent swr and impedance in the region of 75 ohm (depending on surroundings)

but as fullwave dipole on 11m it would be resonant,but fed at a voltage maxima,therefore showing very high impedance.

at approx 7.33m/40.92mhz where it would be 1.5 wave long it would give a low swr and impedance.

i never said you couldn't get a low impedance on it,i said you won't get a low impedance on it at 27mhz/11m,at other frequencies where it is resonant and current fed you will get a low impedance on it.

 

[loosecannon]

marconi,

i think i see what you mean with regards to ANY antenna having only ONE design frequency.
the one point where it has a 1:1 match and is also resonant, meaning zero reactance.

moving away from the ONE design frequency, we can still achieve that 1:1 match but only by adding reactance to the system to compensate for changing resistance.

am i seeing this correctly?
it makes sense as i type it, but my antenna theory is grade school level at best.


as for how the tuning of the A99 and the I-10K go, i dont know enough to tell what is actually happening when you are using the tuning mechanisms.

seems to me, by what im reading here, that all antennas are dipoles, and you cant tune a dipole by only cutting one side of it.

if the point of it all is balanced currents in the radiator and the counterpoise, why do we use 1/4 wave radials on a 5/8 wave antenna?

is this just a "good enough" compromise because it doesnt really matter?

shouldnt we be using 5/16 wave radials on a 5/8 wave antenna?

or .32 wavelength radials on a .64 wave antenna?

or does this "balance" relationship change because of the unique way a 5/8 wave antenna is fed?

yes, i have pretty much all questions and no answers, but i am starting to see a few things that i didnt before; so at least im getting somewhere! LOL

LC

 

[W5LZ]

L.C.,
It may only be 'grade school' level but you've pretty well hit it, and as long as you understand it, who cares what 'level' it is.
As far as the radial thingy goes, if the particular length used is 'compatible' with the impedance matching and the other half of the antenna, it'll work. Lot's of variables in that though. It depends on 'how' your make the things 'compatible'.
Is there a lot left 'unsaid' in that? Of course there is. How do you 'say' it all without writing a dang book? And hope you didn't forget something...
- 'Doc