On Mon, 2006-11-27 at 16:41 +0200, Geoffrey S. Mendelson wrote:
> On Mon, Nov 27, 2006 at 08:29:49AM -0600, Larry Menzel wrote:
> > I have been using receive only, Beverage style antennas for some years now.
> > You are correct in surmising that there is significant voltage on the
> > receive antenna while transmitting. At the contest station near me, W0AIH,
> > who uses high power and significant receive antennas, that has been
> > measured
> > as high as 20+VOLTS and more coming into the receive antenna jack.
> >
> > It would obviously follow that some kind of protection should be applied to
> > receive antenna input.
> ....
> > Just my $.02. OK boys...let 'er rip. What are others doing with this
> > little dilemma?
>
> A popular device with SWL's is a pair of silicon switching diodes, 1N914
> or equivalent back to back across the antenna terminals. The diodes start
> to conduct at about .25 volts, so usually two in series are used.
Last I checked, silicon switching diodes don't start to conduct until
0.6 volts forward. At 0.25 volts, good diodes have megohms of impedance.
If you have 20 volts on a 50 ohm system, you have 8 watts. A pair of
1N914 diodes, or even 4 probably won't survive long running a current of
400 milliamps. Fortunately at that current they will probably fail
shorted, not open.
A diode switch to open the circuit to the receiver input while shorting
on the receiver side of the series diode can do much more to protect the
receiver. It might even work with 1N914 but the intermod will be higher
than using real PIN diodes. Intermediate results can sometimes be had
using 1N4007 which seem to have the I layer in getting the 1 KV PIV
rating. The diode scheme will be faster than a reed relay which takes
several milliseconds so the good reed relay scheme will delay
transmitter enable that long and will delay reconnecting the antenna
until after the transmitter output has stopped. All of which would have
been easier inside the radio than outboard.
>
> An easy way to do it is to take a small aluminum box, mount SO-239's at
> each end and run a wire directly from the center of one to the other.
> Drill a hole in the box and attach a solder lug with a bolt, nut and
> lockwasher. Connect the diodes to the wire and the lug.
>
> Another option is an NE-2 type neon bulb, but the crossover voltage
> may be too high.
Long about 65 volts peak. 43 watts pass through, though at that power
level th NE-2 might melt from excessive power dissipation. Larger neon
lamps were sometimes parts of the input circuit of tube type radios and
sometimes were at the grid of the RF stage where the impedance and
voltage was much higher than at the antenna connection. That connection
isn't available in a solid state radio.
>
> Geoff.
>
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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