Dumb ???? #3 (about radial length)

[wyojeepeer]

I finally got my 30 year old temporary (?) antenna down. I then spent about 2 hours taking apart/breaking the old antenna- a 1/4 wave radioshack with a Francis hotrod 5.5ft as the radiator. After 30 years it did not want to disassemble. Anyway, I then put up another home-brew antenna made from a 7ft Skipshooter and a workman GRK-8 ground plane, with a Caphat at the bottom of the Skipshooter. Measured the SWR, and it's flat at 28.500. That doesn't help me for 11 meters.

I have a Chicom 1/2 wave with three 30 inch radials and was wondering what would happen if I increased the length of the ground plane radials. It screws together with NO adjustments, so, I can assemble it really quickly. And, it's supposed to have an SWR of around 1.2. But, I'm still curious.
This will also be another temporary antenna, until I put up something better. When the weather gets better, and the wind ain't screaming.

 

[wyojeepeer]

Once again, I don't think I explained myself very well.
Is there a relationship between the length of the radiator and the radials?
Can I lower the SWR by lengthening the radials?
How about by drooping them?
My 1975 ARRL book doesn't tell me.

Thanks, John.

 

[AudioShockwav]

with a Caphat at the bottom of the Skipshooter. Measured the SWR, and it's flat at 28.500. That

I can't picture that in my head, did you mount some kind of radials on the skip shooter?
Can you share some photos of the antenna?

73
Jeff

 

[nomadradio]

A half-wave antenna that resonates at 28. 5 is too short. Making it about five percent longer should get you down to 27.2 or so. Back in the bad old days of the "weeping willow" quarter-wave ground plane we would bend the radials up or down to change the SWR AT the resonant frequency. Didn't change the resonance, only the SWR at that frequency.

Your vertical element needs to get longer.

73

 

[Henry HPSD]

hello,

Radials is a story on its own.
To all your questions, the short answer is yes.

The longer anwer would be something like:

All vertical endfed antennas need some type of "radials".
Though for some the demand for it is higher then others.
Again: ALL vertical end-fed type antennas needs some sort of radials.
(this includes the antron 99 types).
Often you dont see it as "radials" as it uses the coax or mast to act as part of the antenna, and some antennas can 'get away" with relative minimal amount of radials. The demand for your halve wave vertical isnt that high ...so there is no need to extend the length.

In some cases the need for those radials are more demanding.
As soon as the current is high at the feedpoint radials will have a large impact on the antenna.
This is almost always the case if the antenna is not a end-fed halve wave.
The reason for that is cause the current is high at the feedpoint.
And on a end-fed halve wave the current is relative low at the end and you can get away with short radials to let that current flow.

How do you know if current is high ?
Well, we know at the end of the antenna resistance is high.. .there is air..
Ohms law tells us : U= IXR.
Now we need to draw the 11 meter "wave" back from the top of the antenna towards the feedpoint.

If R is high that means I current is low and U (voltage) is high again.
here is an example of the the halve wave with the current displayed:

Thats easy...at the end of the antenna.. current is always low so you can draw that wave back to the feed point and find out.

At any place where that current is high demand of radials is high too.
If we take your groundplane... that is 1/4 wave long... we are in the middel of the above picture.. current is high.

All antennas that are not a end fed halve wave ( or any multiply of a halve wave have got relative high(er) current.
That current needs to flow somewhere...so theres a need for radials with 1/4 wave and 5/8 wave kind of antennas.

But its "up to a point".. The first couple of radials have the most impact obviously.

Just think about that mobile antenna without a car.. that wont work efficient at all.

Oke, now if we have a normal groundplane and place it well above earth and has sufficient radials at a 90 degree angle from the radiator the impedance will be ideally roughly 36 Ohms. If change the angle of the radials and put the at a 125 degree angle the impedance will rise to 50 ohms and we can find a nice match !
Thats the reason why lots of 1/4 wave antennas have those radials in an 125 degree angle.

So yes, the angle can change SWR.

If we take that groundplane with radials again at 90 degree.... you could also remove radials... say we have one radial at a 90 degree angle the impedance will rise again from that 36 ohms. The same happens if we shorten the radials... the resonant frequency will shift higher and impedance will rise.

So, yes the length and amount of radials can change SWR.

I dont know the radials you mentioned.
How long are they ? And how many ?

.....Important to remember .....
You could actually tune the 1/4 groundplane for better SWR by reducing the amount and length of radials. BUT you will also REDUCE the efficiency of the antenna, which is not a good thing.
A "worse" SWR in that case is actually a more efficient antenna.

Radial length will also effect SWR, but overall you could say they need to be a quater wave. Why ? well... you want maximum current in that system...and that better with a sufficient amount of radials and a quaterwave (or longer).

The goal is to go for maximum efficiency instead of best SWR.
As you can match SWR...you cant "match" efficiency.

That is unless there is almost no current at the feedpoint...then the radials can be shorter as is the case with your halve wave vertical.

With the above said, there is a correlation between the amount of radials and length. And there is with how high the antenna is placed.
If the antenna is sitting on the ground the need for radials increases.
Thats one of the reason why broadcast MW stations have a hugh amount of very long radials underneath the tower (which basically is a 1/4 wave groundplane).

Kind regards, Henry

 

[BJ radionut]

 

[BJ radionut]

The goal is to go for maximum efficiency instead of best SWR.
As you can match SWR...you cant "match" efficiency.

 

[wyojeepeer]

Jeff,
Imagine, if you will, a 7ft Skipshooter mounted on an "L" bracket (to clamp to a pipe). Between the mount on the "L" bracket and the Skipshooter is a workman WIL-KIT-B six element (about 6 inches long) radial kit that uses 3/8x24 threads for mounting. It's obviously a cap-hat of some kind. It's supposed to help the radiator work better. It don't. (snake oil). Below all of that I put an GRK-8 ground plane kit with 8 elements about 2ft long on the mast. It looks odd as heck. I was curious what the wil-kit would do..... now I know. Nothing good.
Does that help?

The Chicom is a completely
different antenna.

 

[wyojeepeer]

Henry,
If I understood what you said correctly, the only way that the Chicom 1/2 wave can "get away" with using three 30 inch radials is by also using the mast and coax as part of the ground plane. That also allows it to "cheat" the antenna in to thinking it's 50 ohms.
Is that correct?
If I were to increase the length of the radials to 108 inches, what happens? That's the part I don't know. Does it revert to 35 ohms, or what?
If I put a rf choke at the feed point, what happens?

Anyway, thanks.
I never was very good at theory.

 

[Riverman]

View attachment 72763

You like that pic a lot, Gary. I believe this is the 734th you’ve posted it. I use it often. But with just one radial rather than three.

 

[wyojeepeer]

Henry,
If I understood what you said correctly, the only way that the Chicom 1/2 wave can "get away" with using three 30 inch radials is by also using the mast and coax as part of the ground plane. That also allows it to "cheat" the antenna in to thinking it's 50 ohms.
Is that correct?
If I were to increase the length of the radials to 108 inches, what happens? That's the part I don't know. Does it revert to 35 ohms, or what?
If I put a rf choke at the feed point, what happens?

Anyway, thanks.
I never was very good at theory.

I re-read your post for about the 10th time (I'm, slow).
Increasing the length will cause the efficiency
of the antenna to go up, and that's all.
The rf choke should make no difference.
Is that correct?
Thanks, Again.

 

[Henry HPSD]

If I understood what you said correctly, the only way that the Chicom 1/2 wave can "get away" with using three 30 inch radials is by also using the mast and coax as part of the ground plane. That also allows it to "cheat" the antenna in to thinking it's 50 ohms.
Is that correct?
If I were to increase the length of the radials to 108 inches, what happens? That's the part I don't know. Does it revert to 35 ohms, or what?

The only way the 1/2 wave antenna can get away with NO radials at all is by using at least some part of the mast and/or coax as part of the antenna.
Thats why 1/2 wave antennas without radials can work....
I have mentioned that all (vertical end-fed) antennas need some sort of radials.
The demand for those radials are specificly not that high for a 1/2 wave end-fed.
That is because current is relative LOW at the feedpoint.
That also means that impedance is HIGH.
Your end-fed 1/2 wave has roughly an impedance of >2000 Ohms.
Thats the reason there is a coil and most likely some sort of capacitor at the base to transform that impedance down to 50 ohms.

(The 36 Ohms I mentioned was for a 1/4 wave groundplane antenna. So was the part of the efficiency the demand for radials are limited for the 1/2 wave end-fed)

Back to your 1/2 wave end-fed:
If you lenghten your radials on the impedance will change.
Most likley the "imaginary part" of the impedance will become less.
And the real-part will remain more or less equal.
But forget about that...that will make things more complicated.

Just put the antenna up as it is from factory.
It already has some radials, which is good.
There is no benefit to extend those radials, as the halve wave vertical doesnt need an extensive radial network.
You might change the antenna so much the matching network in the antenna wont be able to transfor the impedance of the antenna to 50 Ohms.


If I put a rf choke at the feed point, what happens?

Your radio signal travels from the radio on the inside of the coax towards the antenna.
It is shielded from the rest of the world by the outerbraid of the coax.
It is (tunneld) transported towards the antenna... nice and save with minimal loss.

At the connector it gets to set free to do its work and radiate.
We want that signal (current) to travel on the antenna.
But that signal has no knowledge of "what an antenna" is.
It will use anything (metal) in its proximity.
Unless everything is perfectly in balance that signal will start to radiate using everything that crosses its path.

An end-fed halve wave is far from an balanced antenna.
It would be more balanced is we feed it in the middel.
(Then its a center fed dipole antenna). In that case we have two equal parts...more balanced.
But now it is fed at the bottom... we only have "one" part...
Hence not balanced...
So that signal will radiate and will use anything (metal) to comes across.

That includes the outside of the coax. as now it can travel (back) on the outside part of the outerbriad of the coax.

But that outside of the coax isnt a good resonant part of the antenna.
That can cause some issues, it could effect SWR, if can bring additional noises etc. (as your coax pics up anything on the outside... especially if it runs near houswiring led ligths etc..)
The outside of your coax (and / or mast) will just be part of the antenna.

So we want to block our signal from the outside of the coax or at least in such a way it isnt negative.

One way of improving that is by the using radials.
That can help to "block" the wave from running down.
Another way is to construct a RF CHOKE ...the word says it.
IT chokes the RF signal. its like a "stop sign" for your traveling signal.

It is always wise to use an RF-choke. (despite the amount of radials you have ..despite the type of antenna etc...it is never is a bad thing to use it).
4..maybe max 5 turns and 10cm in diameter and dont tape the choke to the mast but leave it hanging loose away from any metal.

Normally i always say the location should be say 1...1,5 meters below the 1/2 wave feedpoint.
Why ? well.... a lot of 1/2 waves dont have radials...
And we esthablished your signal needs something of radials.
If we you would put the RF choke diretly at the feedpoint you could make performance worse.
But in your case... you allready have some radials ....so that isnt a real issue.
But still, i would put it at say 1 meter below the feedpoint.

Hope it makes some sence...if not ...keep asking !
You are allready doing a better job in trying to understand this then the majority and these kind of things is typical something that is hard to visualize and understand from start...but at a certain point it will all fall into its place.
So hats off to you Sir !

 

[BJ radionut]

You like that pic a lot, Gary. I believe this is the 734th you’ve posted it. I use it often. But with just one radial rather than three.
View attachment 72795

And I'll continue to post , till people quit trying to circumvent physics and find the "easy" no effort way to make an inexpensive and effective antenna.
I do find it interesting that you have time to count how many times I have used that graphic, if you spent that kind of time working on good antennas...what may happen ?...
Oh, I forgot you think quality, effective antennas are UGLY.
I guess you think my home location is ugly then.
All the Best
Gary

PS: The Graphic I use, has 4 tuned radials.

 

[Riverman]

And I'll continue to post , till people quit trying to circumvent physics and find the "easy" no effort way to make an inexpensive and effective antenna.
I do find it interesting that you have time to count how many times I have used that graphic, if you spent that kind of time working on good antennas...what may happen ?...
Oh, I forgot you think quality, effective antennas are UGLY.
I guess you think my home location is ugly then.
All the Best
Gary

PS: The Graphic I use, has 4 tuned radials.

Haven’t counted; was just having fun with you.
Sorry you took it wrong. My apologies.

 

[wyojeepeer]

As for today,
I got the antenna down. Much easier where I have it now, as I can do it by myself. I also took down the mast. Replaced it with a EZ-up 22ft. Now it's even easier.
I took down the Skipshooter, and put up a Semi-duster. That's a Starduster that I drilled and tapped out the radial holes to 3/8-24, so I can use about any whip in America. The radiator remains the same. SWR's are under 2 with no tuning. I'll tune it another day.

Another thing I did was look up the velocity factor for RG8x and IMR 400. I find it interesting that almost no two websites give the same answer. Rg8x was between .75 and .82, depending on the site. IMR400 was between .84 and .85.
I was looking that up because if I put a RF choke on my antenna, it was recommended to put it 1 to 2 meters below the feed point. My thought was having it at a percentage of a wave length would be best. EG :1/8 wave. Have fun trying to figure that out when the VF is all over the place.

John