|
Hi Walter I have been considering the possiblilty of using one
of those 2.4GHz video "wireless-link" thingys. I am not sure how much power they
need or how much signal they would need to give a reasonable dynamic range at
the receiver. You may need to build a full LF TRF RX at the aerial to give
the 0dBm signal that the link really needs. They are quite cheap now, I saw a
new one for under £30 the other day. That's almost cheaper than a roll of coax
!
I think I agree with you if the "reference" is the box the
active aerial is in, then there should be no "height-gain", because one is
sampling the same small segment of the wave front. I was trying to make the
point that a lot of people seem to think that an active whip will work wonders
at ground level, or on the desk, when it really needs getting as high in the air
as you can, to give really useful results at LF.
Cheers de Alan G3NYK
----- Original Message -----
Sent: 05 February 2006 09:14
Subject: LF: {Spam?}
Active_antennas
The theory behind the "height gain"
observed using small LF active antennas at different heights has to do with
compression of the near-earth LF potential gradient caused by the grounded
"mast" holding the antenna.
The essential bit is that there is a
grounded connection between the antenna and receiver, which may be just the
outer of the co-ax cable. It would be an interesting experiment to repeat the
"height gain" experiment without any connection to ground. This could be done
by building an active antenna with a little transmitter to re-radiate the
received LF signal (on 2.4 GHz?) and poking it up using a fibreglass
mast. If the theory is right then there wouldn't be any height gain.
Might do it myself sometime but anyone else interested?
Walter G3JKV.
| 