[Elecraft] Slinky Question

[Ron D'Eau Claire]

Trying to resonate a "slinky" antenna or any antenna when it's in a
confined space near metal can be very difficult. All the formulae are
based on "free space" situations. It's bad enough when the real antenna
is only 40 feet or so from the ground out in the open air. It's a whole
different story inside a building. It can be done, but it is certainly
NOT a simple task without the instrumentation needed to see whether you
need to shorten or lengthen it and by how much. Harmonically-related
antennas (e.g. trap dipoles, etc) are not likely to work well either
since the effect of the surrounding objects varies with frequency. 

The good "slinky" type antennas that I've seen were copper-clad slinkies
hung on a good insulating cord. A short antenna will have a
lower-than-normal radiation resistance. Depending upon how short it is,
the radiation resistance can be down to a couple of ohms. Again, lower
height above ground and the proximity of other objects can reduce the
radiation resistance as well. If the slinky has an r-f resistance of 10
ohms and the antenna has a radiation resistance of 5 ohms, 2/3 of the
power put into it is used up as heat. Put 15 watts in and 5 watts gets
radiated.  Keep in mind that r-f resistance is usually higher than the
d-c resistance of a conductor. R-F only flows in the very outer surface
of a conductor. A very thin-walled tube has the SAME r-f resistance as a
solid bar. So when you are dealing with a so-so conductor like steel in
a slinky, the d-c resistance is NOT a good indication of the r-f
resistance. Add to that the absorbing effect of the material in the
attic and the poor performance of some antennas I easy to understand. 

I was limited to indoor antennas in a variety of apartments for many
years. In one location I noticed that my ability to radiate was worse
than I had grown to expect from other locations. One day I noticed a bit
of a roof tile lying in the garden. It was a scrap piece that a
maintenance man had left behind. Curious, I picked it up and put it with
a glass of water in my Microwave oven. It instantly got very hot. The
composition of the tile obviously was a very poor insulator for r-f. I
had my antenna inside of some sort of "r-f shield" on the roof! 

Still, I had excellent results with QRP in attic-antenna and ceiling
antenna environments. I was pretty successful at being on the upper
floor of the apartment buildings were I lived, and could find access to
the attic crawl space over my apartment. I used simple center fed
"doublet" antennas strung along the underside of the rafters fed with
open-wire line. "Open wire" in this case as NOT the big stuff with the
thick insulators, or the dark brown "ladder line". Being in apartments
with the common "apartment-white" walls, I made my own open wire line
out of the white "wirewrap" wire sold by Radio Shack. From the highest
point of the antenna where the feeders connected, a few bits cut from a
plastic film can provided a spacer for an inch or so. The spacing is NOT
important. What you are after is a fairly high impedance feeder. The
high impedance will keep the SWR down in the range where the losses are
not too great. I arranged to bring the "feeder" into the home right
above the operating desk. An "ice pick" put two tiny holes in the
ceiling right were it met the wall just big enough to feed the thin
white wires through. The holes were invisible from a few inches away.  A
couple of white thumbtacks, push-pins or staples held the feeder wires
to the wall with the right spacing as they came down to my balanced ATU.


Worked a lot of stations on 40 meters from the West Coast with such
antennas using 2 to 10 watts on CW, and a lot of DX on 20 meters. My
antennas tended to be about 50 feet long. The 'rule of thumb' is that
the efficiency of such an antenna is not seriously degraded as long as
the total length of one-half of the radiator plus the feeder = 1/4
wavelength. 

One very common misconception that hear often is that an antenna with a
lot of loading turns it into a non-radiating r-f choke. Some will say
that that if you wrap wire around push-pins to hold it you are creating
an r-f choke that stops the r-f. That's nonsense. The amount of
inductance in a couple of turns of wire around a push-pin might be
significant at 800 MHz, but it's inconsequential at HF. In any event, a
lot of inductance in an antenna such as a "slinky" simply means that it
MAY look "electrically long" at the operating frequency. In the same
way, just making a dipole too long with no loading will make it
"inductive". The solution is to reduce the inductance OR to add
capacitive reactance to cancel the inductance. That's exactly what an
ATU does to reduce the total reactance to zero - along with effecting an
impedance match to 50 ohms for your rig. With an ATU, the only losses in
the system will be those inherent in the ATU and antenna - which might
be significant in a lossy antenna or ATU - but they not caused because
there are any "r-f chokes" in the system. 

Ron AC7AC
K2 # 1289