On Sun, 20 Jun 2010 14:39:27 -0500, Jeffrey D Angus
wrote:
David wrote:
http://www.cliftonlaboratories.com/metal_oxide_varistor_(mov).htm
Amazing coincidence that they act much like the old NE-2 neon
bulb across the antenna leads of old receivers for protection.
They would conduct around 65 volts and suddenly go to near
zero impedance, safely shunting what ever energy on the antenna
line to ground.
And although most receiver inputs couldn't handle a steady state
of 65 volts (or 130 vpp), they could handle them long enough for
the neon bulb to conduct and then shunt them to ground.
We used to test our radios with 117vac on the antenna terminals. While
not a required test, it happened often enough that it was worth
testing and protecting. The AC plug to PL-259 test cable on my cable
rack generated quite a few odd questions.
Neon lamps, MOV's, back to back diodes, PIN diodes, and such are
generally a bad idea in high RF environments. Any non-linear device
between the antenna and the RF amp is going to act like a mixer and
create the dreaded intermodulation products. MOV's and diodes are
particularly bad because they start to slightly conduct at nearly zero
voltage, and increase exponentially with increasing signal. The MOV
also has 100-1000pf of unstable and unpredictable capacitance, which
is not a good thing on the antenna input. The closest approximation
of an ideal protection device are the one-time gas filled spark gaps
used in lightning protectors. No conduction at all until they arc
over. Then, they're dead.
Question:
What's the peak to peak output voltage of a 50 watt transmitter
into 50 ohms?
Answer:
V = 2.828 * 50^2 / 50 = 141 volts p-p
Now, do you REALLY want a device that conducts at 65 volts across the
xmitter antenna terminals?
I actually tried a varistor across the antenna terminals of an HF 150w
PEP xmitter and confirmed the big bang theory.
Hint: Things work differently at 60Hz than at RF frequencies.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558