Antenna current on transsmision line

[bole]

Hi I am making two element VHF vertical antennas with two outputs. Each element is made of 2 tubes 1/4 of wavelength and fed in center, between two tubes. Once I have made first element I have checked for the gain and VSWR and everything was very good, but then I have made second element and as soon as I have passed coax cable thru the first element tubes my gain and VSWR on the first element got affected to the point where I am loosing up to 4dB from the original gain level.

To fix this I was trying all kinds of RF chokes and baluns but I was not able to eliminate negative effect of the second cable passing thru the first dipole. The best results that I have achieved were with cables covered in ferrites but I am still losing 1 dB on the first element as a result of that second cable that is passing thru.

So my question is if anyone has any ideas what else I could try to eliminate currents that are flowing on outside of the feed cable?

 

[mancow]

That description doesn't make much sense. Got a diagram?

 

[bole]

diagram

here is a rough diagram hope this gives you a better picture

 

[prcguy]

Is this an existing design you found or did you think it up?

I'm assuming both coaxial dipoles have coax up the bottom tube which is connected to the outer braid of the coax and the upper tubes are connected to the center conductor with the lower dipole having the coax continuing up through the top tube to the upper dipole.

If this is true I see some problems to overcome like the coax running up through the top element of the lower antenna will greatly affect the tuning of the lower antenna and it may not radiate very well. If you think about a transmission line usually being two conductors with equal and opposite current flowing and canceling radiation, the top tube and outer common mode current on the outside of the coax running up through the upper tube kinda meets the criteria of a coaxial transmission line and the lower dipole may not radiate very well.

The ends of a 1/2 wave dipole are very high impedance and very sensitive to detuning from nearby metal and you have coax a fraction of an inch away as it exits the tube. At the very least you would want to center the coax as it exits both ends of the tubes by making spacers from Styrafoam or some other good dialectic material.

There are some existing designs that start with the basic coaxial dipole and the top element extends for many half wavelengths and there are 1/4 wavelength tubes attached every half wavelength and connected only at the top to create a discontinuity and virtual dipole element every half wavelength.

Check out this antenna which I believe is what I described above: Kreco Antennas - Stacked Co-Axial Antennas

You might even contact Kreco and ask the guy if they run coax up the tubes to each set of dipoles or if they are just coaxial stubs. I've had conversations with Kreco and if you mention your trying to make one as a hobby project they might give you some details. Good luck and let us know if you have any success with the design.
prcguy

 

[bole]

dipoles

Hi thank you for the reply,

" I'm assuming both coaxial dipoles have coax up the bottom tube which is connected to the outer braid of the coax and the upper tubes are connected to the center conductor with the lower dipole having the coax continuing up through the top tube to the upper dipole. "

Yes you got the desig right!

As for detuning i am not very concern about that i can always fix VSWR but you are right about affecting radiation. And yes the current design has cables in center and i am using styrofoam to keep them in center that way the antenna is omni within 1dB.

As for currents on the coax cable that is passing thru the bottom element i know that is my problem but i can not eliminate that issue and i was wondering if someone might have some suggestions.
I know that ferrites on the coax are working, coiling of coax above and below the bottom element works too but it never makes gain level of that dipole identical to top element. There is always at the best 1 dB drop in gain and i am trying to make them exactlly the same gain level. I know i could compromise top element so it gives me less gain and that would make two elements to have same level of gain but i can not do that they need both to have maximum gain as top element.

I will check Kreco and see what they have.

Looking for suggestions.

thank you

 

[prcguy]

I would not expect a problem with the coax exiting the bottom tube of the lower coaxial dipole since that tube is somewhat of an RF choke to common mode current on the feedline, especially if the feedline is well centered as it exits. If there are some common mode currents you can usually slide a bunch of #43 mix ferrite cores up and down the feedline and find the spot that really nulls them out.

I would also not expect the top element to radiate as much as the bottom because some RF has radiated from the bottom antenna and its no longer available to feed the top antenna. A design with separate dipoles fed from power dividers (corporate feed) will split the power equally to each dipole and they will radiate equally, unlike what I see in your example.

I also question the impedance of the bottom antenna feed point since it looks like you are paralleling another antenna and feedline to the same feed point which would reduce the impedance at that point to about half what an individual antenna and feedline would provide.

How are you measuring the 1dB difference between the two connected antennas? I can only imagine a near field scanner would pick that up.
prcguy

Hi thank you for the reply,

" I'm assuming both coaxial dipoles have coax up the bottom tube which is connected to the outer braid of the coax and the upper tubes are connected to the center conductor with the lower dipole having the coax continuing up through the top tube to the upper dipole. "

Yes you got the desig right!

As for detuning i am not very concern about that i can always fix VSWR but you are right about affecting radiation. And yes the current design has cables in center and i am using styrofoam to keep them in center that way the antenna is omni within 1dB.

As for currents on the coax cable that is passing thru the bottom element i know that is my problem but i can not eliminate that issue and i was wondering if someone might have some suggestions.
I know that ferrites on the coax are working, coiling of coax above and below the bottom element works too but it never makes gain level of that dipole identical to top element. There is always at the best 1 dB drop in gain and i am trying to make them exactlly the same gain level. I know i could compromise top element so it gives me less gain and that would make two elements to have same level of gain but i can not do that they need both to have maximum gain as top element.

I will check Kreco and see what they have.

Looking for suggestions.

thank you

 

[bole]

"I would not expect a problem with the coax exiting the bottom tube of the lower coaxial dipole since that tube is somewhat of an RF choke to common mode current on the feedline, especially if the feedline is well centered as it exits. If there are some common mode currents you can usually slide a bunch of #43 mix ferrite cores up and down the feedline and find the spot that really nulls them out."

You are correct ferrites help but do not eliminate problem i have covered both coax cables in ferrites that was the best scenario but still there is a drop in gain on the bottom element due to a presence of that cable passing thru.

"I would also not expect the top element to radiate as much as the bottom because some RF has radiated from the bottom antenna and its no longer available to feed the top antenna. A design with separate dipoles fed from power dividers (corporate feed) will split the power equally to each dipole and they will radiate equally, unlike what I see in your example.

I also question the impedance of the bottom antenna feed point since it looks like you are paralleling another antenna and feedline to the same feed point which would reduce the impedance at that point to about half what an individual antenna and feedline would provide. "

Antenna does not have a corporate feed, no power divider is used here, they are individual elements so two port antenna. They are not combined in parallel they are separately fed.

"How are you measuring the 1dB difference between the two connected antennas? I can only imagine a near field scanner would pick that up. "

The measurement is done outside 35 feet above ground and 40 feet separation between reference antenna and antenna under test. Using network analyzer the gain of single dipole element is compared to gain level of bottom and top element. When gain of top element and gain of bottom element are compared the elevation of bottom and top element are kept same in relation to a reference antenna. The antenna also have been verified in test lab and same results were obtained.

 

[prcguy]

Aah, two completely separate antennas and your just stacking them vertically. I know there will be some interaction and RF induced on the top antenna feedline from the bottom antenna and vertical spacing between them may be critical but beyond that I don't have any suggestions.

I did however just obtain a KRECO stacked coaxial collinear, which I thought is what you were trying to build and am surprised at the simple design. There is an actual coaxial dipole for the top element and the remaining collinear elements are simply 1/4 wave skirts with their tops grounded to the mast and spaced appropriately. There is no electrical connection from the feed line to the lower elements and they are just grounded sympathetic radiators excited by the top antenna.

This would be a very easy antenna to duplicate and I'm excited about receiving it next week and taking some measurements and making some for other frequency ranges. Not that this helps the OP with his project but it gives me more options to build new antennas.
prcguy

 

[kb2vxa]

I see you're trying to make a 1/2 wave vertical dipole but haven't figured out the mechanical design. The lower element is a large diameter sleeve with the coax shield connected at the top, the upper element is a small diameter rod connected to the center conductor. The coax runs up through the mast and the lower sleeve large enough to fit the mast without touching it anywhere but the top.

Since you're probably more confused now than ever you can change your design plans and duplicate the one shown. It's a heck of a lot easier mechanically and electrically identical.