Owen Duffy wrote:

Tony Giacometti wrote in
:

I expressed surprise at the shunt tuning capacitor in an earlier
posting. I know it is a popular circuit, and it features in the ARRL
Antenna Handbook,

and alot of other places also.

Ok, but they are all flawed.

but that doesn't make it a good circuit. Try the variable
capacitor in series with the coax inner conductor, you should improve
the gain by around 20dB.

this one I will try, does this not make the loop a closed loop?

I don't understand what you mean. Just reconfigure the tuning box so that
the capacitor is in series with the inner conductor of the feedline to
the radio. Is that ambiguous?


I guess I am unsure as to how the loop gets connected.

since the loop and cap are currently parallel, removing the cap
leaves me with 2 ends of the loop. Putting the cap in series with the
center of the coax feedline allows me to have only one connection to the
loop.

Did I miss something?





Then try a shunt capacitor on the receiver side of the variable
capacitor, start with 1000pF, you should see further improvement in
gain but with a narrower bandwidth.

Don't have anything like this on hand, this is a variable capacitor I
would assume.

No, just try a fixed capacitor. You could also try 2200pF, it will have a
little more gain but narrower bandwidth.

I have caps in that range, and since its receive only, the voltage can be
low.




This is not a new circuit, you will find it in books, certainly at
least where the tuning / matching network is right at the loop gap.
The relocation of the capacitors by a length of transmission line
does change things a little, and it is more complicated to solve, but
behaviour is soemwhat similar.

Should you try this, your findings would be interesting.

I might be able to get to this in a day or two and I will let you know
what happened.

Thanks.

As soon as I figure out how it needs to be wired.




I was thinking that maybe I didn't have enough capture area with the
loop I currently use.
My next step was to figure out how to increase the size of the loop so
it captures more signal. Since I havn't looked for any info on a loop
like this I was going to ask you if you had anything on a larger loop
but one which isn't too large and can be rotated either by hand or a
rotator.

I think you answered my question(s) at the top of this post.
Doubling the size would still make it small enough to rotate by hand
or with a small tv rotator, matching though might be the other issue
as you mentioned.

In the simple case of an untuned loop loaded with 50 ohms, when the loop
inductive reactance becomes more than becomes more than about 150 ohms,
doubling the size of the loop doubles the induced voltage and doubles the
reactance, so the loop current hardly increases and not much more voltage
is developed in the 50 ohm load. Do you follow?

yes, wellbrook has a large capture area loop, I wonder how they get that one
to work?


A lossless loop of the size you use has a source impedance of 0.0002
ohms. For maximum power transfer, you have to do something better than
deliver a load of the order of a hundred ohms.

It is perhaps a difficult concept to grasp. But it explains why small
antennas don't work so well, it is just so hard to deliver or extract
power because the radiation resistance is so small.

Owen


I never though this was going to be this complicated, I thought all I needed
do was to build the loop and problem solved.......so I thought......