Re: [loopantennas] Re: Stupid antenna tricks
- Why not try a skirting board loop antenna?
In your largest room, run one or more turns of multi-stranded, insulated,
copper wire (speaker cable should be OK) around the outside of the room up
against the skirting board. Or one or more turns around the outside of
the room under the carpet. Because a loop's capture ability is a function
of its area, you get the maximum indoors catch area, in the warm, in an
inconspicuous fashion. A 4 foot loop resonant at about 4 MHz will have a
gain in the region of about -15 dB. A skirting board loop resonant at the
same frequency can have a gain approaching 4 dB. Don't take my figures
for gospel but there is a large difference. If any of the technophiles
on the forum want to post the definitive comparison, I welcome them (I'd
like to know but am too lazy to run the experiments).
The number of turns depends on the perimeter of your room relative to your
frequency of interest plus a number of other factors such as the method of
construction - eg is there iron mesh in the concrete floor?, position of
electrical wiring, plumbing etc. A single turn loop in a room 24 feet by
20 feet in a wooden building should resonate somewhere around 8-9Mhz (This
is a guess - anyone confirm or deny?). It is harder to get that perfect
length than using a multiple turns on a small area loop because each turn
is just so long but the performance benefits are worth the struggle.
Anyway you should aim for your loop to to be slightly shorter than the
resonant length so you can tune it down to your desired frequency with a
variable capacitor. Just connect the variable capacitor to the ends of
the loop wire. If your length is right you will hear an increase in
signal as you adjust the capacitor to resonate the loop at your frequency
of interest. If the loop resonates above the FOI, lengthen
it----resonates below, shorten it. You can, of course, use the loop
untuned by running just one turn and leaving the capacitor out but the
advantages of the tuned loop will be unavailable - just put in a loop with
the largest area. There is no way to predict the actual performance of a
skirting board loop because of the wide variability in the construction
and location of rooms and so some experimentation is needed.
Inside the building you expose your antenna to more electrical
interference. Self-adhesive aluminium foil tape under and on top of the
loop will go a long way to provide some electrostatic rejection. This is
because the aluminium is highly conductive to electricity but has poor
magnetic abilities which allows the magnetic part of the radio signal to
get to the magnetically sensitive loop. Don't make the tape continuous
around the whole loop as it will become a shorted turn and suck all your
signals away - leave a gap of a few inches (at the tuning capacitor is
The tape will alter the resonant frequency of the loop - experiment.
Now you've got to couple the loop to the radio. If your loop is tuned
don't connect the ends of the loop directly to the radio because a tuned
loop is a high impedance device. The input to most radios is a low
impedance and this low impedance will load up the tuned loop and severely
impair its performance. Wind a few turns of thin, insulated wire on a
ferrite rod and connect the ends to a suitable plug using coax cable. Lay
the rod across the loop.
If your loop is untuned, connect the ends to the coax or use a balun
between the loop and the coax. I don't know what ratio balun to recommend
because of the variability in room sizes and FOI but a 4:1 is a good place
Before committing to lifting your carpet or other serious activities, just
place the wire near as near as possible to its final position and do your
experiments/adjustments. Once you've maximised its performance, put it
into its final position, the performance won't change much.
Cost: speaker cable 40c/metre; ferrite rod <$5; plug $1.50; coax
$1.10/metre; aluminium tape - don't know - never bothered; coupler wire
(free - pull an old transformer apart).
> Unfortunately an outdoor resonant antenna for anything other than, oh,
> 11 meters, is out of the question for me. I'm in an apartment and
> while there's a large fenced in backyard where I could put up a
> stealth antenna, it's on the wrong side of the building from my apartment.
- I think my copy of Carr's "Receiving Antenna Handbook" has a very
I've been trying to avoid using a tuner, for the sake of ease in
bandscanning, but I'm not sure that's practical. What I might end up
doing is attaching a random wire and tuned loop to the receiver via a
switchbox. Use the random wire just to check for signals in the clear
and switch to the tuned/tunable loop to improve reception, reduce
As a photographer I've accumulated way too many tripods. I'll
probably dedicated one of 'em to a loop stand. The nearly infinitely
adjustable angles should make it handy for reorienting a loop to
minimize local noise.
--- In email@example.com, "Ray Phillips" <ray@...> wrote:
> Why not try a skirting board loop antenna?
> In your largest room, run one or more turns of multi-stranded,
> copper wire (speaker cable should be OK) around the outside of the
> against the skirting board. Or one or more turns around the outside of
> the room under the carpet. Because a loop's capture ability is a
> of its area, you get the maximum indoors catch area, in the warm, in an
> inconspicuous fashion. A 4 foot loop resonant at about 4 MHz will
> gain in the region of about -15 dB. A skirting board loop resonant
> same frequency can have a gain approaching 4 dB...