|To:||Pete Smith <firstname.lastname@example.org>, email@example.com|
|Subject:||Re: [TowerTalk] Combining antennas|
|From:||Jim Lux <firstname.lastname@example.org>|
|Date:||Tue, 03 Feb 2004 10:51:49 -0800|
At 10:07 AM 2/3/2004 -0500, Pete Smith wrote:
I have a spare Stackmatch box, repaired after a lightning hit, and I'm thinking about using it to let me simultaneously or alternatively feed my 40m yagi at 104 feet, a low 40m dipole running NE/SW at 25 feet, and a medium-height 40m dipole running NW/SE at about 45 feet, in the hope of filling takeoff angle holes for domestic contests. Before I build up the control box, in case I've missed something, are there any reasons why this would NOT be a good idea?
On Tx, you'll be splitting the power among the antennas, according to what their impedances are.
On Rx, it's a little trickier, depending on how the connection is done. Consider two antennas A and B, with no mutual coupling.
Signals received on A will reach the junction, then be divided between B (where they'll either be reradiated, or reflected back from a mismatch) and the radio.
Signals received on B will reach the junction, then be divided between A and the radio.
The net result is that the received power will be halved. However, since, on HF, sky noise dominates, the noise power is also halved, so the SNR will be the same.
Except, the noise from the other antenna is also being summed... What winds up at the radio, then, is
half of the signal from A + half of the noise from A + half of the signal from B + half of the noise from B.
The noise, if uncorrelated (not necessarily a good assumption, by the way), will only sum to 1.4 times the power of each component (i.e. the total noise power will be 0.707 the noise power received by the antenna)
So, the SNR will be 0.5Sa/.707(Na+Nb). If Na=Nb, this is .5/1.414 or about 4.5 dB worse than it was before.
If the noise is correlated, then it will either be as bad as 2x or completely nulled out, depending on the phasing.
If you have a good hybrid junction, you can get more of the power to flow to the radio, rather than the other antenna (assuming everything is "well-matched"). Assuming an ideal hybrid, then, the SNR at the radio will be:
Sa / (.707 *(Na + Nb) ) for uncorrelated noise (a 1.5 dB hit) , or, Sa/(Na+Nb) for correlated noise (a 3 dB hit)
In more practical terms, what you're doing is creating an antenna with more "capture area" with two lobes.
I'm aware that the yagi and the medium-height dipole could couple fairly strongly at that spacing, when the yagi is looking the same way, but think that most of the time the yagi will be perpendicular to that dipole. My modeling of other stacks, when the two antennas are perpendicular, suggests they basically behave like separate antennas then.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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