Problem tuning shielded loop rx antenna ???
Mike Tatum
I'm building a small receiving loop antenna for 160m using 20ft of RG58
coax
erected in a 5ft sq configuration.
I am trying to tune it using a large variable capacitor, it's one I've
bought
for the tank cct of my home brew linear ...
Anyways, the capacitor is connected to the inner conductor of the
coaxial loop with the coax
shield connected together as described in the book I have. I also have
the 1 inch gap in the
shield at the top of the loop as detailed in the diagram.
Now the problem I have is that when I connect my MFJ SWR analyser to the
antenna
and tuune for resonance on 1.84mhz I get a perfect 1:1 SWR by adjusting
the capacitor however,
as soon as I let go of the capacitor / mfj analyser the SWR jumps up to
5:1 ??
Now I guess me holding these two units is causing some sort of
capacitance either from the inner
conductor to the shield or to ground somehow, however I don't seem to be
able to tune the loop
to resonance unless I am making a cct with my hands between the
capacitor and the outer of
the coax.
Can someone please explain to me what is going on here and how it can be
corrected ??
What I am trying to do is get the loop antenna to resonance on 1.84mhz
and then use one of the
-30db nulls to block out strong signals from europe thus hopefully
helping me hear the DX stations better.
Any advice, hints / tips etc will be much appreciated!
Cheers!
MikeT ...
M0AWS
Ps: Please reply to group and cc to me directly as news is somewhat
unreliable here !!
avatar
balanced radiator. In your case it appears that the shield should be
connected to your chassis and ground. The variable capacitor should be
floating above ground. The problem is how to couple this high impedance to
the radio's 50 ohm unbalanced input. There are several ways to do it, but
the simplest, once the preliminary adjustments are made, is to use a gamma
feed.
Dave
W0MAY
Mike Tatum <mi...@cybatek.fsnet.co.uk> wrote in message
news:3A5776E5...@cybatek.fsnet.co.uk...
Richard Clark
Hi Mike,
You might need to decouple the drive point from the drive line with a
1:1 current Balun. Of course there may another problem here with
stray capacitance (which would mean a problem of a ground path) as you
have already noted. It may also denote you need to feed with a
capacitive divider (one of many feed configurations) since your body
capacitance to ground seems to be a key component (if it in fact does
not reveal the isolation problem that would be solved with the Balun).
73's
Richard Clark, KB7QHC
Reg Edwards
receiver as with a magloop - using a smaller loop, 1/5th or 1/4 the
diameter of the main loop.
---
Reg
--
=============================
Regards from Reg, G4FGQ
For free radio design software go to:-
http://www.btinternet.com/~g4fgq.regp
=============================
Alan Melia
earth potential) I think so you need a length of insulated spindle to get
your body capacity away. You didnt say how the RX is to be connected. The
usual is a coupling loop. Setting it for min SWR does NOT tune it though!
You would be better to put a small loop on your MFJ analyser and use it as a
signal source to couple a signal into the RX loop. Then tune the loop to a
peak on
the RX. The null is at right angles to the plane of the loop so depending
where you are it may be difficult to null out Europe if say you want the
States. The nulls are sharp and the peaks broad so there is something to be
gained usually. Even quite small loops works quite well, I have used a 4
turn 14inch diam device on 160m and 80m mainly to get rig of TV tb. The
signal will be 20dB or so down on a wire but the noise will be down as well.
So hopefully the S/N on the required signal will be better. The depth of the
null depends a lot on the balancing of the loop and not injecting signals
into the loop from the coax feed to the RX. You may also need to detune any
wire aerials nearby (i.e the TX wire) as they will inject noise and signals
into the loop.
I use a 4 ft loop quite successfully for RX on 136kHz, and can hear most
things.
Cheers de Alan
Alan....@btinternet.com
Mike Tatum wrote in message <3A5776E5...@cybatek.fsnet.co.uk>...
Mike Tatum
Thanks for the info.
I am reading this design from an antenna book I have purchased,
The author details that the feed coax from the RX be connected directly to
the loop,
that is inner to inner and outer to outer, there is no mention of any type of
matching
circuit in the article ?
I will now go away and read up on gamma matching !
Cheers!
MikeT ...
Mike Tatum
Hi Richard!
Thanks for the info.
According to the article I am getting the design from, the author details
that the
50ohm feed coax just be connected directly to the loop, that is inner to
inner
and outer to outer, there is no mention or even detail in the diagram of any
type
of matching component, just a direct connection.
A current balun makes sense though, I'll read up on how to make one and then
have
another go at tuning the loop.
Cheers!
MikeT ...
Mike Tatum
Will a loop of this sort of design still have the two deep nulls that I
am after ?
The book I have does talk of this type of loop antenna, but says nothing
of the deep
nulls that the other single loop design talks of.
MikeT ...
Mike Tatum
Thanks for the info.
The rx is connected directly to the loop using the same RG58 coax, that is
inner to inner
and outer to outer, exactly as detailed in the authors design and indeed
diagram.
I've tried the insulated spindle, it does get rid of the problem of the stray
capacitance
however the SWR will not drop below 5:1 when checked with the MFJ VSWR
analyser.
It looks like I need to work on isolating this loop completely from the ground
system, it's abviously
getting a connection back to ground via the RX which is connected to the shack
earth system which in turn
connects to the earth radial system outside.
Another article I have read talks of having too much capacitance in cheap RG58
coax making the antenna impossible
to tune ?? I am now also wondering if this is a factor I had not thought of ??
Back to the drawing board I guess :)
I've had a few other replies and things to try so I have plently to experiment
with.
Cheers!
MikeT ...
Reg Edwards
of the loop. The greater the isolation of the loop from feedline and
ground connection the sharper the null.
Using 2 or 3 turns allows a tuning capacitor of only 1/4 or 1/9 the
size. No need for a screened loop. Just use hefty wire. It will be
more efficient and more simple to construct.
For design of multi-turn receiving loops download in a few seconds
program RJELOOP3 from website below.
--
=============================
Regards from Reg, G4FGQ
For free radio design software go to:-
http://www.btinternet.com/~g4fgq.regp
=============================
Mike Tatum <mi...@cybatek.fsnet.co.uk> wrote in message
news:3A584E5A...@cybatek.fsnet.co.uk...
Cecil
Hey Reg, this is off the subject but why are Land Rovers in England
so heavy? I'm watching a TV program of cop chases. They said the
English cops were chasing a 15,000 pound Land Rover.
--
http://www.mindspring.com/~w6rca
W4JLE
"Cecil" <Cecil....@IEEE.org> wrote in message
news:3A60BD8F...@IEEE.org...
Brian Kelly
Cecil wrote:
> Reg Edwards wrote:
>
> Hey Reg, this is off the subject but why are Land Rovers in England
> so heavy? I'm watching a TV program of cop chases. They said the
> English cops were chasing a 15,000 pound Land Rover.
>
12,000 pounds of it was decent Cognac.
>
Cecil
> Prodding the old fecal matter yet again?
All in good fun, Fred.
--
http://www.mindspring.com/~w6rca
Reg Edwards
The sooner we shift to Euro-dollars the better.
***********************************
Cecil <Cecil....@IEEE.org> wrote in message
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