On Wed, 1 Aug 2007, Paul Tomlinson wrote:
> As a newbie to this band, I'm wondering how best to have to adapt my
> existing HF antenna to work on 500KHz or make a new one .... any help
> please would be greatly appreciated.
Hello Paul,
As another newcomer to LF/MF I've been having great fun learning a new
way of looking at antennas and transmitters in order to get on 500kHz.
For an antenna the accepted norm is a vertical, as tall as physically
possible, with some "top-loading" to increase the capacity and increase
the radiation resistance.
These aerials are always going to need some form of inductive loading to
resonate them and some form of ground system to work against to increase
the efficiency. The ground can take the form of radials (as many as
possible and as long as possible - the key seems to be to get as many
directly in the vicinity of the base of antenna as possible) and/or
earth rods.
My initial antenna was made by running a wire up a 7m fibreglass fishing
pole held up with guys and a top loading wire about 12m long running to
a support on the chimney. This is also referred to as an "Inverted L"
but the main radiating/receiving element is the vertical section.
I resonated it initially by winding a coil of around 1.5mH on a pice of
4" soil pipe and tuned by a variable capacitor of around 0-500pF -
tuning for max noise on receive. This arrangement is unsuitable for
transmitting due to the high voltages developed across the high
reactances of the tuning components!
I heard plenty of signals (and saw more on QRSS-3) from my location in
the Shetland Islands so I decided to apply for a NoV and built a
transmitter.
The loading coil was then replaced by a variometer of about 500-800uH
range to remove the need for a variable capacitor and a tapped ferrite
matching transformer to allow me to both tune for resonance and match to
50 ohms for the TX.
For guidance about LF/MF antennas I found ON7YD's guide (meant for
136kHz but applicable to any LF/MF application) to be extremely useful.
It's available here :-
http://wireless.org.uk/on7yd
As for receiving - many normal amateur rigs are a bit deaf down at
500kHz so unless you've a good communications receiver that is well
specified for MF and below, some sort of pre-amp or receive converter
might be a good idea. I use an AR7030 which works very well.
I wonder if you've heard anything on 500 yet, even with just your normal
HF antenna? Signals might be weak but as long as you're withing a
hundred miles or so of a few stations you should here something -
especially if you try software like Argo or Spectran to "observe" the
band - this is a good way of seeing very low level signals that you
can't hear aurally.
Good luck and keep asking questions - that's what I did and I've now got
my NoV and have had a couple of contacts. Further TX and antenna
work put me off the air for the last 2 weeks but I'm almost ready for
another foray.
Cheers,
John GM4SLV
Shetland Is
IP90gg
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