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

[Riverman]

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.

 

[Henry HPSD]

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.

Im afraid that has litlle use, but it is a good way to start a myth...

First thing would be the velocity factor:

- The common mode currents run on the outside of the coax.
THERE IS (besides some material loss) NO VELOCITY factor.

Secondly, the ideal distance of the Choke depends...on the number of radials the thickness and length of them etc.
Best option would be to attach one or more radials on your half wave end-fed
And place the Choke at a distance of 0.1 ..0.15 lambda from the feedpoint.
The ideal length varies a bit but longer then 0.15 is never a good option.

More importantly would be to make sure the choke impedance is high enough ( you could measure this with a nano-vna or similair).
If you dont have one...4 turns 10cm diameter and leave the choke hanging ...dont attach it to metal.

Kind regards, Henry

 

[wyojeepeer]

of 0.1 ..0.15 lambda

OK, got everything but that...... What the heck is a Lambda????

I made an assumption that the VF would have an effect on the coax. I don't see that it doesn't. Are you saying that coax shield has no VF? How can that be? And if it does, then wouldn't having a fraction of a wave length make sense?
As you have an RF wave going through the antenna, is that not effected by the material of the antenna. Steel, copper, and aluminum all resonate at a different frequency, (please read that as VF) why would the coax braid be different? after all, isn't it a part of the "counterpoise".....

Still confused....
John.

 

[Henry HPSD]

OK, got everything but that...... What the heck is a Lambda????

I made an assumption that the VF would have an effect on the coax. I don't see that it doesn't. Are you saying that coax shield has no VF? How can that be? And if it does, then wouldn't having a fraction of a wave length make sense?
As you have an RF wave going through the antenna, is that not effected by the material of the antenna. Steel, copper, and aluminum all resonate at a different frequency, (please read that as VF) why would the coax braid be different? after all, isn't it a part of the "counterpoise".....

Still confused....
John.

Keep asking with whatever is confusing you.
It most likely is my bad explination, Im trying to learn about antennas everyday...so I take some things for granted....which obviously they are not always good to get things right !

Lambda is the same as wavelength.
(Or I believe it is the word for the greek capital letter λ which is the symbol for wavelength.)

Yes, a coax cable has a VF...you mentioned that 0.75..0.8 previously.

That VF is for the signal travelling inside the coax.
One of the reasons it has a velocity factor is because the inside of the coax isnt `air` but the space between the inner conductor and outer conductor made from non guiding material ....its an isolator...to seperate those two. (you dont want a short circuit). The signal has different speeds for various materials.
One of the reasons the VF can be different.

But you are trying to block a signal on the outside of the coax cable.....not the inside... on the outside of the coax there is a litlle bit of plastic and air...
So the velocity factor isnt what you were trying to find out (that .75 ) but it would be more in the order of .98
The outside of the coax doesnt have a velocity factor (besides the material loss ....(velocity factor) as previously mentioned)

Yes ....materials can have effect...
But that is minimal....you wont notice the dfference between aluminium and copper.
When both have the same diameter the resonant frequency will be the same.

Diameter in general has far more influence on length.
For example:
A resonant halve wave antenna with a 2mm diameter wire has a different length then a 20mm diameter resonant halve wave (for the same frequency).
It is very easy to reach more then 1MHz difference with different diameters on 27Mhz. That also means the length of that resonant antenna will change significant....difference in the order of 4..5 inches are not uncommon
(again for the same antenna ..same resonant frequency...just different diameter).

Hope this helps to get a better idea?