1/4 wave groundplane & end fed verticals - time for some FACTS

[Moleculo]

Remember this statement from the article:

Many antenna designs actually use the feedline and mast radiation that others dismiss as "insignificant" to increase antenna gain. In some cases, the antenna designers really don't even understand what they did to create a "Magic antenna".

So yes, it's possible that if this is occurring then under the right set of circumstances it might make more gain. Think of it almost like a cophased vertical. The problem with this though, is that it's difficult/impossible to control where the feedline radiation goes and the results will vary a lot from installation to installation. Some will be lost to ground. Some will go into your neighbor's stereo. Some will radiate all back into your shack and get into your computer, mic, other equipment. You get the point. So if the feedline radiation isn't going where you want it to go (out on the horizon), then it's not doing much good, is it?

 

[74IN]

How can common mode be detected?

 

[Moleculo]

How can common mode be detected?

If you want to measure it, here's a simple clamp-on meter you can build that will work. This looks like a fun project

Clip-on RF Current Meter

 

[Splash1]

74in....heres a real simple way to tell if you got common mode currents....grab your coax whilst yer checking your swr and if yer needle moves,,you got common mode currents.

 

[Marconi]

74in....heres a real simple way to tell if you got common mode currents....grab your coax whilst yer checking your swr and if yer needle moves,,you got common mode currents.

Splash1, your recomendation is widely touted in CB as the right way to detect CMC's, but the author Moleculo links us to in his thread, W8JI disagrees. See his quote below:

W8JI ...

Common Mode Currents

At high power 3 dB loss in even the largest components would mean extreme heat, at low power its difficult to notice several dB loss as any type of component heating. The paradox is while 1500 watt systems could often stand to lose 10 dB or more as heat...low power systems (at least in my way of thinking) should try to squeeze every milliwatt out!

Let's look at a matching system in what I consider one of the most difficult methods of feeding an antenna, the end fed half-wave. If anyone has any other matching systems that are commonly used or recommended, send me one and I'll measure it in my lab and post the data here.

There is also some discussion of common mode current, and the lack of common mode current because we sometimes can't observe ill effects.

I remember working with a new graduate engineer, let's call him Simon, on an antenna system. When I asked Simon if he checked for proper feedpoint isolation, he turned an SWR analyzer on and wrapped his fingers around the feedline. Seeing no change in SWR, he declared the system free of common mode currents! Convincing him to use a clamp-on RF meter, we found the feedline that acted "cold" to the touch actually had significant common mode currents.

Where did Simon go wrong? Pretty simple when we think about what he was actually testing. High voltage points interact with body capacitance at HF, not current points! Had Simon grabbed the coax at a high voltage point of the shield, he might have found interaction with SWR. Unfortunately a significantly high impedance or high voltage point rarely appears along the outside of a "grounded" shield. That's because the cable is often routed near other conductors. The cable is also thick, and that limits surge impedance.

Even if we modify common mode impedance with body capacity, the feedpoint is where multiple paths combine and make the transition to the feedline. Measuring SWR changes is a very poor way to determine proper operation. Even the most basic antenna systems, once the feedline becomes involved, can become a terribly complex web of paths. The myth that we can grab a cable to see if a system needs a balun or has common mode problems is a very BIG myth.

The whole of the article above is located at: end fed 1/2 wave matching system end feed

How to make a clamp-on RF meter is also included as a clickable link in the body of this quote.

 

[Marconi]

BTW, antennas like the Interceptor 10K (and others) do have a proper ground plane system. Models of well designed 5/8 wl antennas with proper radials will be different than those shown above.

Moleculo, the only CB antenna models I have ever seen published on the Internet were for the Imax that is noted in the W8JI link you provided. You mentioned modeling of well designed 5/8 wave antennas with proper ground planes. Could you give us the links?

Since you make a distinction about the I10K having a proper ground plane, what is it you find particularly proper? It’s not even symmetrical to the radiator and the large mast bracket surely has some ill effect as well. The Wolf .64 GP is similar with the elements spaced even further apart, and that is probably not good either. Among several other brands and models, at least the Maco and the Merlin have symmetrical GPK attached.


I can't tell you how much difference this all makes to antenna performance, but the lack of symmetry and balance is reported as very important---at least where CMC are concerned.

 

[W5LZ]

Marconi,
"I can't tell you how much difference this all makes to antenna performance, but the lack of symmetry and balance is reported as very important---at least where CMC are concerned."

I can't tell you either. But since that CMC is produced by the voltage/current of the signal, then it is taken away from what should/would be radiated in the desired way. That 'desired way' could/would also touch on that "lack of symmetry and balance" part.

I also don't know how to quantify those qualities, how much is 'too much' for any particular type/size of antenna. Or what should be considered 'normal' or acceptable for whatever size/type of antenna under some particular set of circumstances. I have 'messed' with certain 'types' of antennas for long enough that I have a sort of 'feel' for what's 'normal' for the particular circumstances I've had to work with (my backyard, and the particular antennas I've used).
Naturally, with the limitations I've had to work with, I can't speak from personal experience about a lot of antennas. But, having 'made do' with a few less than 'optimal' situations, I've got an opinion about certain aspects of antennas, in general, that are probably fairly close to being 'factual', for lack of a better way of putting it. I think that's about as 'normal' as you can get, and holds 'true' for just about everyone who has played with antennas for very long.
Having sort of set the 'stage' with my experiences (or lack of them), here are a couple observations that I've found to hold up in most circumstances.

Don't get too obsessed with any one particular part/aspect of an antenna. Depending on circumstances, no antenna will always be the best in all of them.

The description 'goundplane' has got to be the most confusing name for a 'part' of any vertical antenna. It's never adequate to describe anything with out a 'qualifier' or two added to it, 'elevated', real dirt, or whatever. It's easier for me to understand if it's called the 'other half' of the antenna, whatever it requires to perform adequately. (I was introduced to antennas with a dipole, and they make 'sense' to me. So other antennas are typically referenced to them, and when done so, usually make 'sense'. Is that the best way of thinking about antennas? Beats me, but it 'works' for me, so it's the best for me.)

Higher is better to some absolutely ridiculous degree. That 'works' for any antenna, starting at about 1/2 wave length and ending at something around 1/2 mile.

A 'too long' antenna is easier to deal with because adding capacitive reactance to make it the 'right' length is sometimes easier than adding inductive reactance to make the thing longer. Or, just cut the @#$ thing off some. Capacitors tend to be smaller than coils.

The best excuse I can think of is, "If I only had _____!". Insert any word you feel necessary, room, stuff to work with, money!, higher tower, etc, etc. It's also the most useless excuse I can think of.

The hardest part about evaluating any experience (your's or someone else's) is the thinking part of it. "So what did I/they do 'wrong'?", sort of thingy. Sometimes that 'wrong' thing stands out like a naked/nude female. And sometimes it's the absolute last thing I wanna see (depends on the female). (If you pick the gender correctly, that works for both males and females.) A 'tacky' way of thinking about it, but I'll bet it will stay with you, huh?

If you can't have the 'full sized' antenna, do some 'modifications' to it to make it work. Very seldom do those 'modifications' ever make it better than the 'real thing'. Using 'seldom' instead of 'never' is a CYA thingy, by the way. 'Never' is the word to use, but I'm CYA'ing.

'New' and 'improved' descriptive wording is advertising. Treat it as such unless factual proof is offered. Evaluate that proof before accepting it. No matter what 'they' are talking about, that 'new and improved' thingy will only be very slightly improved. You'll be very lucky to be able to tell any difference. Maybe.

And having dispensed all the 'useless' thoughts I've had lately, I'll quit. Apply liberally, spit out what doesn't fit.
- 'Doc

 

[Moleculo]

Moleculo, the only CB antenna models I have ever seen published on the Internet were for the Imax that is noted in the W8JI link you provided. You mentioned modeling of well designed 5/8 wave antennas with proper ground planes. Could you give us the links?

I didn't say there were "well documented" 5/8 wave CB antenna models. A 5/8 wave is a 5/8 wave is a 5/8 wave on any band if they're constructed the same. Search for 5/8 wave antenna info and don't restrict yourself to CB stuff.

Since you make a distinction about the I10K having a proper ground plane, what is it you find particularly proper? It’s not even symmetrical to the radiator and the large mast bracket surely has some ill effect as well. The Wolf .64 GP is similar with the elements spaced even further apart, and that is probably not good either. Among several other brands and models, at least the Maco and the Merlin have symmetrical GPK attached.

The ARRL Antenna book tells us that a 5/8 wave vertical only needs to have radials that are 1/4 wave long. That doesn't mean that they can't be longer, though. 'Doc is right (and knows more than me, too) about the symmetrical part. We're talking about HF here, not UHF. Ask yourself, how much of a wavelength are the radials on the I-10K or Wolf apart from each other? The I-10K is what, one inch (if that) apart? That's not even worth typing a sentence about on HF. If you think it is, can you show me some data or testing that says otherwise? Sometimes on HF the structural integrity of the antenna is more important than perfect symmetry, especially on some minor detail like this. When big broadcast LF stations put their AM towers up and spread out ground radials, do you think they make sure the earth is perfectly level and exactly symmetric in all directions?

About capacity hats: Sometimes cap hats do more than just allow a shortened antenna to work better than it would without the cap hat. For example, on my mobile Hi-Q, one of the benefits of adding a cap hat and keeping the antenna the same length I get to take a bunch of the loading coil out of the antenna, thus reducing the loss significantly. If you look at the specs published of the I-10K, it says that it's vertical length is 247" at 28.5 MHz. That's exactly 5/8 wave vertical length, according to my calculations (you can verify here Antenna calculations). So in this case, the length of the antenna is not being shortened by the cap hat. It must be there for some other purpose.

Marconi, if you want the antenna models of the I-10K, you're going to have to ask Jay for them. I know that he's done it on EZNEC and also some other real expensive software that he has access to that most (nearly all) hams and CBers do not. You can figure out for yourself what your chances are on getting copies of those models are.

I'm not claiming that the I-10K or Wolf or any well designed 5/8 wave antenna is the perfect antenna (is there such a thing?). There are circumstances where a 1/4 wave groundplane might work better to suit a specific need than another type of antenna. I don't claim to understand all there is to know about antennas. But I think I understand enough to recognize when it's being done wrong.

 

[CDX-007]

Excellent, another believer in Charlie's genius! Can't beat a Hi-Q, why do they keep trying?

As far as the length on 10m @ 28.500 mHz, I come up with 265.25" for a .64, (not a 5/8 or .625) which is what the I-10K is advertised as being.

I know the I-10K has about 12" top-hat radials so there's still a few inches hiding. Perhaps it's in the Velocity factor, or perhaps having four top hat radials causes a bit of a capacitance effect, making them appear longer to RF...?

When I rebuild a Penetrator I dump the top-hat and lengthen the radiator to 279.5" for Channel 20, or 266.2" for 10m (28.400 mHz). The MFJ-259 usually shows zero reactance with the new 12 gauge copper beta match parts I fabricate. I've never had an issue with corona discharge but I never run more than <2K through it on 10m.

And on 10m I tune it for 28.400 mHz - center of the Novice / Technician band, not 28.500 mHz.
- Seems like most of the activity (when the band is open) is around 28.365 - 28.425 mHz.

Hey! - There's a Taylor Grandslammer GLR-4, like Eddie (Wolf) & I both had when we were teens, on ePay right now for $350, rebuilt like new.
It's cool to see the old Girl. Brought back memories to see that hub design.

Check it out: Taylor GLR-4 "Grand Slammer 6400" CB Radio Base Antenna - eBay (item 230331444548 end time Apr-14-09 19:14:35 PDT)

Back in the 80's I tried the Penetrator without the 4 ground radials at the base of the antenna to see what would happen to the overall performance.
One thing I noticed was it lost all it's horizontal receive and acted like the Imax did. When someone went horizontal they basically disappeared, but with the 4 ground plane radials on the Penetrator when someone would go horizontal they would only drop about 10dB.

73

 

[Moleculo]

As far as the length on 10m @ 28.500 mHz, I come up with 265.25" for a .64, (not a 5/8 or .625) which is what the I-10K is advertised as being.

We're starting to nit pick I think, but the Interceptor is advertised as a 5/8 wave, not a .64: Home of the Interceptor 10K Antenna. Also check the spec page for the advertised length on 28.5Mhz. I think the Wolf was advertised as a .64 wave?

 

[CDX-007]

We're starting to nit pick I think, but the Interceptor is advertised as a 5/8 wave, not a .64: Home of the Interceptor 10K Antenna. Also check the spec page for the advertised length on 28.5Mhz. I think the Wolf was advertised as a .64 wave?

Well Buddle my Bisterlitch! You're absolutely correct. He does refer to it as a 5/8 and not a .64. I stand (surprised and) corrected!

And as the instructions are to set it to 247" to the 12" top-hat that would be just what I calc at 28.5mHz, 259".
247" radiator length plus 12" for the top-hat makes 259".

Hmmm, I wonder why he doesn't go the full .64 which is supposed to be the highest gain point possible?

Since the I-10K is so adjustable, I wonder if someone who has one could try extending it to the full .64 (266" to the top-hat for Ch.20 - assuming the top-hat radials are each 12" long) and then reset the match for 50ohms, zero reactance, and see if the performance is at all noticeably improved.

 

[Marconi]

Well Buddle my Bisterlitch! You're absolutely correct. He does refer to it as a 5/8 and not a .64. I stand (surprised and) corrected!

And as the instructions are to set it to 247" to the 12" top-hat that would be just what I calc at 28.5mHz, 259".
247" radiator length plus 12" for the top-hat makes 259".

Hmmm, I wonder why he doesn't go the full .64 which is supposed to be the highest gain point possible?

Since the I-10K is so adjustable, I wonder if someone who has one could try extending it to the full .64 (266" to the top-hat for Ch.20 - assuming the top-hat radials are each 12" long) and then reset the match for 50ohms, zero reactance, and see if the performance is at all noticeably improved.

The top hat is to make the tip larger which acts to discourage lightning strikes. I think Jay states this, more or less, on his Website and/or in his documentation. The top hat on the I10K seems to be too small to really act much like a top hat in my opinion. If it fell off, I doubt one would even notice it due to regular operations.

 

[hookedon6]

The top hat is to make the tip larger which acts to discourage lightning strikes.

the entire radiator is @ DC ground.

a larger tip on any verticle does help to reduce any "static/corona" dis-charges that tend to create noise when they discharge.

 

[mackmobile43]

the entire radiator is @ DC ground.

a larger tip on any verticle does help to reduce any "static/corona" charges that may tend to build up.

Would a small loop @ the end if the vertical work as well as a corona ball?

 

[mackmobile43]

The top hat is to make the tip larger which acts to discourage lightning strikes. I think Jay states this, more or less, on his Website and/or in his documentation. The top hat on the I10K seems to be too small to really act much like a top hat in my opinion. If it fell off, I doubt one would even notice it due to regular operations.

Do you think the hat on the .64 wolf is large enough? If not I'll make mine bigger.