GHOST ANTENNA STORY

 Year 1991. I got posted to Defence Electronics Application Lab, Dehradun. I was living on 1^st floor of a 3-storied building. We had a small balcony facing Mussoorie hills, about 18 Kms aerial distance away. I had HW32a, Heath-kit 20M SSB mono-band transceiver, which I had modified to cover full 14.000 to 14.350 Mhz range. For CW operation, I used to send keyed 800 Hz tone through mike! I also had Heath-kit HM-102 to monitor SWR /Output Power.

Since roof top of this building was not easily accessible, I thought of putting up a small helical wound dipole antenna with capacitive hats on ends. Such type of antennae has a narrow BW and requires frequent tuning to cover the entire band. Also, off-centre loaded dipoles have a larger BW as compared to centrally loaded ones. So I decided to have an off-centre fed helically wound small antenna with capacitive hats for an overall larger BW.

So made a small helically wound antenna using #18 enameled Cu wire, with a feed point at about 1/3^rd from one end. Since I had no idea about the feed point impedance of such a contraption, I started with connecting a 4:1 ratio ferrite core transformer at the feed point of antenna. Cable used was RG58A/U.

 After tweaking HW32a for maximum output power, I started making SWR measurements. After adjusting the forward power to full scale, when I put the switch of SWR meter to reverse position, the meter reading did not move at all! I thought the reverse switch connection has become open, so opened the meter and checked all connections and the switch. Everything was OK. Started checking the SWR again. Same result. Removed the antenna cable; SWR gave same reading for forward & reflected power – as it should. Again checked the SWR meter by connecting dummy load of various values and the SWR meter read perfectly well!

What was puzzling that the reflected power was not there at all!! Even with a perfectly matched 50 ohms dummy load , the meter needle moves a little, maybe very small deflection but you can always perceive the tiny movement. In this case there was no movement at all – as if the reverse power switch has become open! It was like that antenna contraption was SUCKING power, no reflected power at all!!

 Then started checking the SWR across the entire 20M band – same result! Very astonishing! Thought this antenna is behaving like a very lossy wide band device with perfectly matched impedance and would hardly radiate any signal.

But in the absence of any other explanation, thought why not give a CW call and see what happens. It was mid noon, 20M band is always dead at this hour; you don’t hear even a single station. But I got totally shocked when on my first CQ call, a VK station responded and we had a nice but short QSO! My hands were shaking with excitement, mind spinning around in circles!!

No way - I thought. It is against all theory, so rechecked the complete setup all over again. Same results!

I had before me a frequency independent power-sucking antenna!

I decided to verify the results by dismantling the entire setup and rebuilding all over again. So quickly de-soldered the antenna cable connections, removed the 4:1 matching transformer, removed the SWR meter and changed the position of antenna cable.

I did not note down any connection details as it was a simple case of putting a matching transformer at the feed point. That’s where I committed a blunder! When I started re-soldering the matching transformer in 4:1 configuration, I could not connect the de-connected wires in a natural way. I had to readjust the position of the connecting wires by pulling and bending them in a different way to make the proper connection! I immediately knew that earlier I had connected the input transformer in some other way by mistake! Also I did not note down the position and the number of turns of the coiled excess antenna cable.

Anyway, checked all the connections and started measuring SWR. Now the antenna was behaving as it should! High SWR, its value changing as I changed the frequency and what not!

Tried soldering the transformer in all possible configurations with no results! I could not repeat and reproduce the results again!!

I was not new to electronics that time. I have done B.Tech & M.tech in Electronics & Communications from IIT – Delhi. That time I was working on a secure Frequency Hopping 200 hops/sec Spread Spectrum Software based VHF transceiver with in-built speech secrecy for DRDO – which was later inducted and used by our forces as world’s most secure transceiver!

 I am a firm believer of matching practical results with theoretical values. It may sound ridiculous, but I surely did observe this phenomenon.

I can conclude the following:

 * It was not a case of Impedance matching.

 * This contraption was sucking/coupling power from the transmitter to the space in a perfect              manner. There was no effect of changing the input frequency.

   The same was true for reception.

Sharing this story, without bothering about the comments, because I know this happened!!