To: rsgb_lf_group@blacksheep.org References: <007001c62a34$97e808b0$2101a8c0@AUG2004> Date: Sun, 5 Feb 2006 11:57:25 -0000 MIME-Version: 1.0 X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 5.00.2615.200 X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2615.200 Subject: LF: Re: {Spam?} Active_antennas Content-Type: text/html; charset=windows-1252 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-PN-SpamFiltered: by PlusNet MXCore (v2.00) Content-transfer-encoding: 8bit

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 -----

From: Walter Blanchard

To: rsgb_lf_group@blacksheep.org

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.