[Lowfer] Loops on 185.3

[Mitch Powell]

> Have you tried some of the hard line for the loop?

I built one 6-footer as per the Mitchell Lee article - using hardline, and
it worked well. Obviously, the larger diameter, the greater the signal. I
then went from 6 - 8 to 12 footers...... and have also built a 25 ft square
one ( using this same concept - untuned) - but the noise is rising
faster than the signal at that point - so I found that the 12 ft is about
optimum.

> So the loop couples to a toroid a-frmr which feeds the input inductor
> of the preamp?
> Is that a 1:1 toroid winding?

Did you read the two articles ( Lee and Burhans ) on the tele410 site ?
They explain the transformer input, etc.


> What about the "missing coupling cap." Is that the unmarked input to
> the 2N3904?    

Yes - that is correct - from the RF choke to the base of the 2N3904.
 
> I've only know of Burhans amps as high impedance input amps.

That is not correct, as Ralph did a lot of work on this low impedance
matching, with simple - readily available transformers.
Take a look at the articles.

1 - The interior loop is a single turn of wire - I use a piece of RG58/U,
leaving the insulation on - and then use the shield as the
conductor.... making a nice fat, low resistance wire.

2 - being very low impedance - the input xfmr is an inexpensive audio xfmr
(costing < $2:00). Pri = 200 ohms CT, Sec = 8 ohms CT. And you will notice
the secondary used as the input - and the primary as the coupling to the
input filter and FET. So, low input impedance and some step-up to the
amplifier. Also, the transformer has a natural roll-off just above 500 kHz
so the higher frequencies are attenuated nicely. The addition of the simple
LPF between the xfmr and FET cuts off very sharply at about 600 kHz.
The power gain is > 25 dB and has proven to be very stable through all kinds
of weather. I have never lost an amplifier due to lightning, or proximity to
rf - even though I have run 300 watts within a few feet of the loop.

The xfmr is available through MOUSER (Mansfield, TX)


3 - The output xfmr is reversed for impedance matching - and matched the
output of the amplifier to the 50/75 ohm coaxial line to the receiver.
The +12 Vdc is fed down the line, using a capacitor and RC filter. I can add
that circuit if you are interested.

4 - at the receiver I have a 1:1 toroid (T50-75) for isolation. It does make
a difference.

I hope this helps answer some questions.

By the way - the loop is tight inside the branches and trunk of the maple
tree - and certainly not in the clear. I use a nylon rope as an "Armstrong
rotator" to swing it from IP and NC and WA.   It's fun at night, with a
flashlight - and neighbors wondering about it all. However, over the years
they have gotten used to the strange backyard configurations. and usually
end up helping ! 

73
Mitch  VE3OT