[UK-CONTEST] More about DX loops

Wed Apr 6 16:07:45 EDT 2005

Very puzzled Callum. You have 6 runs of coax to 6 different (monoband?)
antennas. Yes?

Disconnect the coax at each antenna, measure the impedance of the antenna
with your analyser at the feed point and then you can work out what matching
your require. You can either use the transformation properties of a
quarter-wave section of transmission line or an L-match

1 A transmission line can be standard 75 or 93 ohm coaxial, 300/450ohm
ribbon or even 600 ohm open-line. There is a formula which gives
transmission line impedance for a given source and load impedance. I don't
have it to hand.

2 L match networks are mechanically simple so no need to bother with an ATU.
You can get thin bore copper tube from B&Q and some decent capacitors are
always available on e-bay. I recently won a bid for a wide-spaced 400pF
variable for nine quid. There are spreadsheets available to work out the L
and C values and there is a formula for working out number of turns of an
air spaced coil which I found pretty accurate.

I currently have a mixture of both here, all at the antenna feed point :
1 - 30m diamond loop base fed with a quarter wave section of Belden coax,
(Vf about .85 against usual .66)
2 - 40m pair of phased 1/4 verticals. L-match, 400pf (big) mica and I think
about .6uH (half of former Butternut HF2V base inductor) in the box
containing the phase switching relay.
3 - 80m GP. 53ft vertical with L-match at base. Coil about 6 turns 1/4" D
copper tubing, 3 inches diameter. 500pF variable at half mesh.
4 - 160m dipole at 55ft is about 30 ohms I recall, minimum SWR 1.7 and 2.5:1
+/- 30KHz. I recently designed and built an L match with a 2x500pF broadcast
variable and about 7 turns 1/4x1/8 inch copper strip. It transforms the SWR
to 1:1 and less than 1.5:1 +/- 30KHz. I sited it at the base of the mast,
which also houses a 6 way relay box for one 100m W103 coax run back to the
shack.

As the Yanks say : "Do the Math"  Chris G3SJJ

-----Original Message-----
From: uk-contest-bounces at contesting.com
[mailto:uk-contest-bounces at contesting.com]On Behalf Of Callum
Sent: 06 April 2005 17:15
To: uk-contest at contesting.com
Subject: Re: [UK-CONTEST] More about DX loops

Guys,

I have 6 sets of Westflex 103 running from the shack here at the front of
the house to the loft and then into the garden etc. If I assume that these
loops have roughly an impedance of circa 100 ohms, what's the best way of
transform this to 50 - because I won't be able to "ribbon" from the shack to
the loops.

So, there's two topics now..(!) Feeding horizontal loops at the house (I
have two) and feeding the field day vertical loop. I'll feed the field day
one at the corner, but ribbon? That's slightly outside my sphere because
I've never used it. Can I not cheat and use coax like I was taught on the
foundation course!! ?? :)

BTW, I've now been lent the low band DX book, so perhaps the answers are in
there - however Chris, Donald, Clive and Adrian (and Darren - nearly missed
you!) all seem to have slightly more knowledge about this than me.

(Apologies for waffling about antennas and not contests!)

Callum
M0MCX

-----Original Message-----
From: G3SJJ [mailto:g3sjj at btinternet.com]
Sent: 05 April 2005 10:56
To: uk-contest at contesting.com
Subject: Re: [UK-CONTEST] More about DX loops

I agree with that one Clive. I had some heavy duty (power rating) 75 ohm
ribbon from Westlake but recently the plastic casing cracked in several
places after a number of years use. Apparently they aren't stocking it any
longer as they would need to buy a 1000m drum and there is insufficient
demand. Chris G3SJJ

-----Original Message-----
From: uk-contest-bounces at contesting.com
[mailto:uk-contest-bounces at contesting.com]On Behalf Of Clive GM3POI
Sent: 05 April 2005 11:09
To: Callum; uk-contest at contesting.com
Subject: Re: [UK-CONTEST] More about DX loops

Callum,
The best way to feed a loop that is also light is to use a quarter wave of
75 ohm ribbon. That will transform an expected Z of 110-120 ohms down to 50
ohms where you can introduce a 1:1 balun. Alternatively you could also
corner feed it and not bother with a quarter wave down one side. From
modelling you would see it is still very much vertical polarization, I did
this on my old 160 delta loop that worked rather well.
73 Clive GM3POI
----- Original Message -----
From: "Callum" <callum at mccormick.uk.com>
To: <uk-contest at contesting.com>
Sent: Monday, April 04, 2005 4:58 PM
Subject: [UK-CONTEST] More about DX loops

> Thanks to Alan and John who both mentioned similar aspects of loops for
> DX -
> also to Adrian who brought the topic up..
>
> In my mind, I have a loop for 80 meters, slung off the 100 foot trailer
> tower at a club field day event. I was going to use a 4:1 balun, however
> perhaps I could get away with a few turns of coax? 4:1 might be too steep.
>
> My dilemma is that if I feed this 1/4 wave down one of the legs, the
> weight
> of the balun / coax / feedline etc will pull the delta loop into an odd
> shape. 1/4 wave "down" is just about a third up from the bottom (I'll work
> it out - this is just for example!) - I wonder the best way of supporting
> this, or perhaps I use a heavier gauge wire than I normally do and just
> pull
> it tight to take the "bend" out? I don't want to snap the balun etc. I
> built
> a 40 meter vertical delta loop for CQWW last year and fed it like this,
> but
> I supported the weight by managing to take the coax off at 90 degrees to
> an
> upstairs room so there wasn't mush real weight on the thing.
>
> Any ideas on the engineering side, how I might do this as it's really
> nagging me?
>
> Callum.
>
>
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