Topband: Newbie K9AY observations & decoupling

[George (K8GG) & Marijke Guerin]

Frank, VO1HP wrote:

----- Original Message ----- 
From: "VO1HP" <fdavis at nfld.net>
To: <topband at contesting.com>
Sent: Monday, October 13, 2008 8:28 PM
Subject: Topband: Newbie K9AY observations & decoupling


>I have had a K9AY in operation now for a couple of weeks. Using the
> WX0B control and relay boxes and the antenna is fed with RG6 with F
> connectors. It is 40ft east of my 52ft tower and 160M coaxial
> Inv "L". I built the two loops from scrap wire and fibreglass tent
> poles ..height 25ft.  The NE loop is made from insulated 14ga. house
> wire and the SE/NW loop from enamel coated 14ga copper.
>
> It may be my imagination but the enamel coated loop always sounds
> more "alive " then the other one. In either case there is plenty of
> signal coming to the FT1000MP so that I rarely use the on board
> control box preamp. Have placed four 25ft radials on the grass under
> the loops and bonded same to a 4ft copper ground rod that was
> successfully pounded into our flint like earth.
>
> The NE loop is actually oriented a bit to the east of north so the
> panel marings on the control box don't actually lineup with the
> orientation of the two loops.
>
> No other antennas closer to the loops.
>
> I notice that directivity of the loops on the BC band from below
> 1600khz is dramatic to say the least. Eu BC stations on SE loop
> for are practically unreadible on the NW (west) loop. Likewise USA
> BC stations on SW loop (west) and gone completely on the east loop.
> The loops appear to work properly on BCB freqs which are far away
> from the resonance freq of the 160M Inv L.
>
> On 160M however directivity of the loops is not dramatic at all.
> Many signals are the same strength on all loops.   Some signals from
> Eu appear to be stronger on the East loop but not all the time.   USA
> stations appear to be same on NE and NW loops. THe switching of the
> termination resistors appears to be working as I can notice chages
> iin F/B when I vary the resistors while listening to BCB stations.
>
> So I am concluding that the tower and the Inv L are screwing up the
> loops on 160M.?      THe loops are actually near  the ends of several
> radials for the "L". I have tried decoupling the Inv L at the shack
> end of the Inv L coax by alternatively opening it and also grounding
> it to the station ground....but see no difference on loop rec'd
> signals.
>
> Have not yet tried decoupling schemes right at the base of the "L".
>
> Also I am not yet using a common mode choke on the RG6 feedline of
> the loops. I do have a "choke" at the base of the "L"...consisting
> of 12 turns of RG58 on a 2.5 inch toroid of unknown specs.
>
> Anyone have any suggestions on the best (easiest) way to decouple
> the "L"?
>
> 73 Frank VO1HP
>

Frank:

1.  For decoupling the TX antenna:
If you are using the K9AY loop system only on 160 meters, then a relay to 
disconnect the inverted-L wire from the feed point on receive will "float 
the wire" and it will look like a half wave on 80 but will not be resonant 
on 160.

2.  For decoupling the feedline at the K9AY RX loop system:
If I remember correctly, the feedpoint transformer for a K9AY loop is a 9:1 
or 6:1 UnUn, and if the feedline winding is isolated from the coil in the 
receiving loop, then, theoretically, there is no need to decouple the 
receiving coax at the control/relay box.  You can check that with an ohm 
meter from the shield side of the F-connector to the wire loop connections 
and ground connection on the box.  If it is grounded to the radial system 
(and loops), it would be good to put some ferrite sleeves on the RG-6 at the 
feedpoint.

3:  A question for you:
Why are you using a feedline choke at the inverted-L antenna?
If the center conductor is feeding the antenna, either directly or through a 
matching network, and the shield is connected to the ground radials, then I 
do not think a choke helps.

73 & good luck

George    K8GG