[Fwd: LF: Antenna Experiments]

[Andre' Kesteloot]

Mike Dennison wrote:

> I have used a variety of antennas on 136kHz and have recently
> been using a 55ft (17m) vertical with some inductance half way up.
> This was an effective antenna and noticeably less directional than
> the inverted-L which I used before (the L had a null towards the
> horizontal section). Unfortunately it suffered from not being
> mechanically stable enough and this meant that whenever the wind
> was blowing, it waved about looking dangerous so had to be
> lowered. More importantly, as it moved it went out of tune - Rik
> said my signal was verying between S2 and S4 during a recnt
> QSO, and I could see the current changing.
>
> Last Saturday, I put my old inverted-L back again. This has a 30ft
> (9m) vertical section going to a tree branch and a 60ft (18m)
> horizontal section going to a pole.
>
> Results with this antenna were initially about the same as the
> vertical. I then put the loading coil in circuit at the top of the vertical
> section and got a 3-4dB improvement in reports. The coil is about
> 100 turns on a drain pipe former. I still need some bottom loading
> but adding top inductive loading does help.
>
> Positive:
> The inverted-L is much more stable. I did have a problem with the
> nylon string insulator melting (it was wet) and the antenna came
> down twice but I fixed that with better insulation. Note: essential to
> use a halyard (continuous loop) when hauling up antennas on a
> tree.
> It works well. Most reports are up - G3KEV gave me 2 S points up.
>
> Negative:
> Because the end of the horizontal section is near the houses (the
> 55ft vertical was 70ft (20m) from any house) I get S9 noise from
> next door. This noise is not on all the time but was on almost all
> weekend. I had to use my 1m loop antenna to receive anyone.
>
> Conclusions:
> 1) Putting some inductance at the top of the vertical section of an L
> does make an improvement.
> 2) An 'L'  9m+18m with top inductance works better than a 17m
> vertical with centre inductance (though the vertical works amazingly
> well and will be used by me whenever I go portable).
> 3) Having any part of an antenna (even the bit with the most volts
> on it) near a house is bad news for receive.
> 4) Wet nylon string is not an insulator.
>
> I also altered my receiving loop. I had tapped the coax a couple of
> turns up the 'coil' and got good results. However, better results and -
>  most importantly for my purpose -  deeper nulls were obtained by
> adding a balun so that the loop operated in balanced mode. I could
> have tried a separate coupling loop instead but ran out of time.
>
> All in all, a most instructive weekend's tests.
>
> Mike, G3XDV (IO91VT)
> http://www.dennison.demon.co.uk/activity.htm