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From: hajo.brandt.dj1zb@t-online.de
To:  <rsgb_lf_group@blacksheep.org>
Date: 22 Jul 2009 13:51 GMT
Message-ID: <1MTcEO-1SeuiO0@fwd10.t-online.de>
Subject: Re: LF: Loop (was Re: IGBT in 136 KHz TX?)
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Dear John,

I am sorry, but I do not see your problems. I do NOT tune my loop. The
loop is feeding a transformer, its secondary is connected to a cable
into the shack to the receiver or the preselector. The inductivity of
my
loop is 30uH;
forming an triangle with lengths of 2 m, 7,5m and 7,5 m, the 2 m
vertically near the house, the other end is fastened to a tree. Minimum
height of the loop is about 2,5 meters, just that people can walk
below.
That is all. I am pleased with this design for receiving and do not see
any needs for improvements. 

For comparison I have set up a two turn triangle loop made of twin-lead
mains cable, each side 1,80 m in length, which also gave an inductivty
of about 30uH, to fit to the same transformer. As its area was only 1/4
of the larger loop I have not been surprised to see that the received
VLF to MF signals were two S-units down, compared to the larger loop.

I can also listen on VLF to MF using either a
passive T-antenna via a 9:1 unun or an active antenna and make
comparisons. As most of VLF to MF activity here in Europe originates
north of my QTH I am using the untuned loop pointing to the north in
most cases. 

I should add that I am living in a rural area. In spite of this the
T-antenna downlead had to be placed at least 5 meters from the house,
for not picking up noise from the mains. The unun is placed below the
downlead and grounded there, and a cable is going into the shack.

73 Ha-Jo, DJ1ZB


"John Andrews" <w1tag@charter.net> schrieb:
> Ha-Jo,
> 
> That's one way to look at it, and a valid approach. But consider another 
> example:
> 
> Suppose you have a single-turn loop that has an (arbitrary) impedance of 
> 0.1 +j10 ohms at a frequency of interest, and you desire to make it look 
> like a 10-turn tuned loop. A 1:10 transformer will provide the same 
> voltage step-up as a 10-turn loop, and will ideally step the impedance 
> up by 10^2 = 100, giving you 10 +j1000 ohms. You would then have an 
> option of series or parallel tuning the loop on either side of the 
> transformer. The secondary side is much nicer for our use than the 
> primary side. Parallel tuning would be a good choice into a hi-z input 
> preamp. Series tuning would leave you with the 10 ohm resistance, which 
> you could then step up to the characteristic impedance of your line, or 
> leave at a lower value if BW were an issue.
> 
> In the above case, I'd suggest a primary inductance on the transformer 
> of 5X the magnitude of the loop impedance, but your 4X would probably 
> do. Leakage reactance is pretty much inevitable, and will throw things 
> off a bit.
> 
> There's all sorts of ways to approach this, depending on your desire for 
> tuning, etc. But I just wanted to point out that the much simpler single 
> turn construction can work just as well as multi-turn.
> 
> John, W1TAG
> 
> 
> hajo.brandt.dj1zb@t-online.de wrote:
> > Dear John, LF,
> > 
> > I would like especially to comment on using a one winding loop combined
> > with a step-up transformer:
> 
>