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Coupled mode coaxial cable antenna

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quixotron

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Greetings,
I am working on converting a coupled-mode coaxial cable into a standing wave antenna. If anyone is not aware of it, it is a coaxial cable that has a corrugated copper shield and apertures milled into the shield at very close separation distances. The purpose of the shield and apertures is to provide an impedance mismatch and the discontinuities in the shield allow for the presence of standing waves. Since the magnitude of the standing waves decrease with radial distance, this cable becomes an inductively coupled Tx line. Its radiation pattern is that of an endfire antenna, see Antenna Theory: Design and Analysis 3rd edition pg 550.
In US patent no 5,473,336 entitled, cable for use as a distributed antenna, the author states that the coaxial cable is open ended and periodically loaded with some type of outer shield conductor tape melded onto the dielectric filling.
I was thinking that a dipole is a surface wave antenna, in that the arms end in open circuits, therefore the current is zero at the ends and there is a reflection of +1, the voltage wave is reflected and in phase with the sending wave, thus doubling the the effects on the electric field. But this is an electrical antenna, suppose I need a magnetic antenna, shouldn't I then short-circuit the load in order to get twice the current and doubling the H field strength? Would that burn out my amplifier or radio?
So whats my question? I have tested the leaky cable for coupling loss. This is the ratio of the power received by a dipole to the power in the cable. Technically I used a loop probe antenna, and I measured the leaked power on a portable spectrum analyzer. I also measured the power from another loop antenna. It seems the loop radiates more power than the leaky cable. I tested the cable by opening and short-circuiting the load. I did receive a boost in about 10 dBm. but it was still short compared to the loop.
I next ran an rf power amplifier through my Txing end. I received a -XdBm level peak compared to a -X+10 dBm on my loop. I can operate certain devices with the loop, but the cable seems dead. Also, for a -X dBm peak, I had to insert 40-50 watts of power into the cable. This is unacceptable.
However, I am stuck as how to improve the power being emitted by the cable. I need the power output from the cable to be greater than or equal to the loop. Right now I am measuring the input impedance of the cable and its S21. So, does anyone know of a procedure to improve the leaky power? I know in some cases, you can improve the gain of the cable by extending its length several lambda, but this is not possible in my case.
If anyone can help me, I'd very much appreciate it.
Thanks,
 
Update: Yesterday

f=900 mhz, 50 ohm load.

Alright so today I took an S21 measurement of the cable. However, rather than doing a straight response, I terminated one end of the cable while varying the load-open, short and 50 ohm. The second port of the VNA had a loop probe antenna that I used to "sniff" points across the cable, the left end, center and right end sections. Being an endfire antenna, I expected for there to be higher losses at the ends. I did this for another loop antenna as well.
The best resonse came from the open circuited cable.
It turns out the open circuited cable had a S21 response of X dBm at the left end, X+5 dBm, and X-7 dBm on the right. The loop was at X+20 dBm. The cable is definitey not radiating as much as the loop. I then remembered in a WIFI antenna design that a coffee can was used to improve the gain of a robe antenna by 6 dBi. I therefore, wound copper tape in a helical fashion around the cable. It boosted my S21 response by +6-7 dBm.
I am still too short on the S21. I will see if I can place the cable in a iece of coper tubing, maybe that will boost my gain.
 
Update: Today

I found a 1" diameter copper tube and aluminum tube. I placed the cable inside the tubes. I received a better response with the aluminum tube and the same response with the copper tube.

This is strange, shouldnt the tubes prevent rf leakage, since they are closed tubes, not including the endpoints? I was thinking of cutting apertures in the tube to allow for additional RF leakage and more pronouncing standing waves.

If I open circuit the load, we get a +1 reflection. in this case the voltage wave doubles since it adds in phase and the current goes to zero. I am thinking if the tubes need to be smaller in diameter, since the electric field might not be strong enough to induce a voltage onto the coupling tube due to the radial distance between the outer conductor and the tube.

I'm defnitely going to take more math classes and electromagnetic courses in school.TEM theory is far more complicated than folks lead it to be.
 
Update: Today

wow! You guys aren't much help. neither was the rf forums!

I've solved the problem, and I ain't sharing! :)
 
quixotron said:
wow! You guys aren't much help. neither was the rf forums!
Possibly because it was not obvious what you were trying to do, and not many of the users of this forum are RF experts.
But as you say, RFglobalnet with all its RF experts did not take you on either.

If it is any consolation, I looked up the patent which you cited in your first post and it seemed to relate to a modified leaky feeder antenna which was characterised to only radiate over short distances, quite intentionally.

quixotron said:
I've solved the problem, and I ain't sharing!
OK, dont share the solution, but it would be nice to know what you were trying to do with this cable/antenna.

JimB
 
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