LF: 500 kHz antenna

[Rik Strobbe <rik.strobbe@fys.kuleuven.be>]

Dear all,

I am looking for short term antenna and TX possibilities on 500
kHz.

For the antenna I have 2 options:

1. a T-antenna 7m high and 10m top
Simulation with MMANA gives a 0.13 Ohm radiation resistance. On
137kHz the loss resistance was about 140 Ohm (a lot of trees), based on
Jim's information I expect that it drops to about one third (+/- 50 Ohm,
how convenient) on 500 kHz.
The "gain" should be somewhere around -20dBb.

2. a loop 7m high and 15m long
Simulation with MMANA gives a 0.005 Ohm radiation resistance and 0.35
Ohm copper loss (using 4mm wire).
If there are no additional losses the "gain" is about -16dBd.
But the loop would be slung through trees and at the lower side about
0.5m above ground. Any idea what additional losses are to be
expected?

The loop will be in east-west so the null will be north-south (sorry
Gus), what would be OK for the UK, most of Germany and Central Europe on
surface wave. I guess that for high angle ionospheric propagations the
nulls will disappear more or less

Figures tell me that loop is as good or better that the T-antenna. Anyone
to support that or to advice me otherwise ?

For the TX I would like to adapt my 137 kHz TX (G3YXM-like design) to
500kHz. This would give me enough power to get 1 or 2 Watt ERP with a
small antenna.
Any experience in that direction ? 
In particular: 
- what is the efficiency using IRF350's (on 137kHz I get > 80%)
- values for the output filter (2 or 3 stage Pi-filter)
- I use a TV HV-transformer core at the output (for balancing and
matching to 50 Ohm). It works well on 136 kHz, but on 500 kHz ?

An alternative would be to use my TS440. I know that cutting 1 diode
allows it to transmit over the entire range, but can it be used as low as
500 kHz ?

73, Rik  ON7YD

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