It looks like the Microvert antenna may have originated in Japan? The Microvert is a very compact antenna. Some believe the coax is a major part of the radiating system. Having never tried this sort of antenna I am unable to comment. This was posted to the Microvert Yahoo group earlier:
Whether you believe it's the coax radiating or not, this is still interesting:
The Super Rad Antenna (Induced Secondary Radiation Antenna)
There are other websites about SRA (secondary radiation antenna), but unfortunately all in Japanese. Its definetly a Japanese thing!
I have all the parts required here to make one of these. Whether you believe its the coax radiating or not the results will be interesting.
Rob
M0RZFPS. My old 10m/12m modified microvert gives a good SWR.
Hi Roger,
ReplyDeleteI don't think they work as claimed. The feed coax plays a very large part in the radiation of any RF.
See my 40m antenna shootout near the top of this webpage.
http://www.g8jnj.net/currentprojects.htm
In this test I was careful to decouple the exposed 2m length of coax from the base of the antenna by means of an elevated radial system and good choke balun.The rest of the feed coax was buried.
The coil is wound on a conductive tube so it is a shorted transformer. There may be a little leakage inductance which you can resonate out with a series capacitor. You get the same result by shorting the end of the coax, it then covers all bands without needing tuning!!
ReplyDeleteI spent some time with GM3HAT years ago, he was a house guest several times, we spent many hours talking about his work. In all of the "demonstrations" the feeder was hot.
Maurice used to move toggle switches on his phasing device by using a cork as an insulator, if you touched the metalwork you received a substantial jolt. This was with 10W of RF and a short path to a real ground. That happened with all of his CFA's et al.
Some feeder radiation is almost inevitable with all real antennae in real life unless you choke or screen the feeder. Even with a very good dummy load and good screened cables some RF leakage happens, that's why we need screened rooms for making some measurements.
But unless you go looking for feeder radiation you will not normally be worried however that also means that the feeder can pick up a lot of noise when it is close to home wiring- but then people assume that it has been picked up by the antenna!
Many people now report very much higher noise levels but they also connect PC's to their rigs which are wideband noise generators. One way of reducing the noise level is to wrap several turns of the antennae feeder through a ferrite ring. That has worked very well for me on LF,MF & 160 on Rx.
Alan
G8LCO