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LF: Re: RE: Re: RE: loops

To: [email protected]
Subject: LF: Re: RE: Re: RE: loops
From: "Dave Brown" <[email protected]>
Date: Sun, 31 Mar 2002 10:19:36 +1200
References: <[email protected]_wil08>
Reply-to: [email protected]
Sender: <[email protected]>
Hi  Bill
The  aluminium stuff  is two runs, each one having 7 strands of  1.5 mm dia
Al  - the drop-lead stuff - there is a very similar looking 'product' that
is much more common as it has a steel centre strand and is used for the long
runs up and down roads etc.  More physical strength but----  It has an outer
insulation jacket (only the drop-lead stuff has this-the steel strand stuff
has no insulation) and both runs are cable tied together all round the loop
proper.
It was the best I could come up with at the time I set the loop
up-certainly far from ideal but has proven to be a useable compromise.  I
may try something else if a suitable quantity of something better turns up.
Both runs are paralleled at the ends where they are fed, which happens to
be in one bottom corner of the loop.  I use a transformer to feed it- uses a
pair of old B&W TV line output cores with 14 turns on the primary and 2 on
the secondary for a nominal 50 ohm feed point.
The loop is not very big- about 50 feet long and 25 feet high with the
bottom about four feet off the ground. Inductance of around 64 uH.  It is
down the back of the yard behind garage and other out buildings. A
top-loaded vertical was just not practical here as the yard is so small.
But bigger things are afoot -- elsewhere!

My LF linear uses a pair of 813s in passive grid (200 ohm resistor from
grids to ground-xfmr coupled down to 50 ohms input with 'yet another' LOPT
xfmr core) with drive derived screen voltage ( as per G2DAF linear, if
anyone remembers it!) - about 50 watts of drive gives me a measured 800
watts out on SSB- I use the 813s with the graphite anodes- the more usual
metal plate anode ones won't go the distance at all!
Have to throttle things back a bit on  near 100 % duty cycle modes- QRSS
etc- more like 500 watts out.
This gives me comparable performance up and down ZL to a top loaded vertical
of similar height  on an average sized yard with only 100 watts in the
antenna. The price for using a loop I guess. But it IS totally non-critical
as regards weather etc. Tuning is on the nose (very narrow 27 dB return loss
null around 180 kHz) almost regardless of anything. I managed to shift it
about 100Hz putting a pipe against one of the sides to see what effect it
might have. Somewhat brutal!.
Cheers
Dave, ZL3FJ


----- Original Message -----
From: "Ashlock,William" <[email protected]>
To: <[email protected]>
Sent: Sunday, March 31, 2002 7:11 AM
Subject: LF: RE: Re: RE: loops


Hi Dave,

Many thanks for the comeback with you loop parameters. Have a few
questions
about these:

>Loop conductor is two runs in parallel of 7/1.5mm aluminium-- your bog
>standard 11 kV overhead drop-lead stuff.

I assume this is two strips of 7mm x 1.5mm Al? The two are running in
parallel, but assume they are they also connected in parallel at their
ends
rather than in series?

>and effective resistance at 180kHz of 0.99 ohms - a bit on the high side,

I suppose the skin effect losses are pretty high with a 1.5mm thickness
since this is about 10 times the skin depth. This is what makes larger
diam
solid or stranded conductors not worth much (unless Litzed.) What are the
dimensions of your loop and how high is the lower portion above ground?

Bill A



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