[Lowfer] SWR meters for LowFERs

[Bill Ashlock]

Hi Lyle,

I used to think that the relatively large size of these loop antennas would 
result in problems for harmonic generation. However this is not the case. I 
have checked the spectrum up to many multiples of 185k and find very little 
signal - far less than I measured back in the old vertical days. The Ferrite 
core (type 43 or 77) that we use to couple our loop antennas to the 
transmitter appears to act as a darn good LP filter for elimination of upper 
harmonics. I also believe the system Q of 100 to over 200, depending on the 
loop conductor, limits the current at the upper frequencies to a greater 
extent than it does for the Lowfer vertical because the multiple resonances 
of the vertical due to spray capacities are not as prominent.

The std NPN/PNP switching stage works very efficiently when fed directly 
into the coupling transformer as long as the bases are not overdriven or 
underdriven. In fact my go/no-go test for efficiency is to contact my upper 
lip to either plastic transistor and check for any heat at all. The ideal 
output impedance for maximum efficiency is 0 ohms and the load resistance 
(RL) seen by this stage is simply the loop AC resistance multiplied by the 
square of the transformer's turns ratio. Matching of the transmitter to a 
transmission line (and transmission line to antenna) in the traditional 
sense (standing waves and all) is not used, and the RMS power delivered by 
the switching stage is close to Vcc/8RL.

Would be overjoyed if you were to give these loops a test at your base!

73
Bill


>From: "Lyle Koehler" <lyle@mlecmn.net>
>Reply-To: lowfer@mailman.qth.net
>To: "lowfer reflector" <lowfer@mailman.qth.net>
>Subject: [Lowfer] SWR meters for LowFERs
>Date: Sun, 28 Jul 2002 11:11:52 -0500
>
>Bill Ashlock is correct in that there is no need to worry about SWR in the
>usual sense, because in any good LowFER installation the transmitter is
>located right at the antenna. But to obtain efficient power transfer from
>the transmitter, the load should be resistive rather than reactive. The
>impedance should be neither very high nor very low, to avoid excessive
>voltages or currents in the final. Something in the 50 ohm vicinity is 
>about
>right, and an SWR bridge is perhaps the simplest homebrew instrument that 
>an
>experimenter can use to determine when the antenna is presenting a 50 ohm
>resistive load.
>
>As for a filter in the final output, it may seem unnecessary if you're only
>interested in meeting the -20 dB Part 15 requirement on out-of-band
>emissions, because the antenna itself is very narrow-band. But we also have
>to remember that at higher frequencies, the antenna is much more efficient
>than it is at LF. For example, a 15 meter loop of #10 wire has a predicted
>gain (at 8 degrees elevation) of about -30 dBi at 185 kHz; -15 dBi at 500
>kHz; -5 dBi at 1 MHz; and -1 dBi at 1.5 MHz. Similar numbers probably apply
>to vertical LowFER antennas, although they come in many shapes and sizes. 
>At
>any rate, if the fundamental signal from my beacon can be heard at 1000
>miles, I feel that the harmonics should be down a lot more than 20 dB in
>order to avoid interference to some licensed radio service. So I'm willing
>to pay the penalty of a fraction of a dB that might result from the
>insertion loss of a properly designed filter working into a properly
>designed antenna matching network.
>
>Lyle, K0LR
>
>_______________________________________________
>From the Lowfer mailing list
>Send messages to: Lowfer@mailman.qth.net
>To sub/unsub visit: http://mailman.qth.net/mailman/listinfo/lowfer




_________________________________________________________________
Join the world�s largest e-mail service with MSN Hotmail. 
http://www.hotmail.com