- July 31, 2020 at 6:58 am #115455dosmanParticipant
Total posts : 8
I’m actually very surprised my login still works here, lol. And glad this place is still around while the “other hobby broadcaster forums” still take themselves waaaay to serious. Hoping to fire up my old part-15 rig after a few years of absence after a transient static spike took my transmitter out. It was an easy fix once I had the time to dive into it, just a couple new transistors in the output finals.
That is all, thanks.July 31, 2020 at 9:34 am #115460MarkModerator
Total posts : 602
Sorry to hear about the transmitter.
You can protect the output from just that by installing a coil across the output from antenna to ground. The value if it’s an FM transmitter is 2.2uH. For AM I don’t know if that value will work as the different frequency range. It has to be a value that has a very high resistance in several megohms to the frequencies transmitting but see a short at DC.
There may be a site with a inductor calculator where you could get the right value for AM.
My AM transmitter already has the protection on board in the form of a gas discharged arrestor which acts like an MOV.July 31, 2020 at 9:45 am #115461dosmanParticipant
Total posts : 8
Awesome, thanks! It’s AM but I can look into what value coil to use as a surge arrestor there.August 1, 2020 at 6:42 pm #115467radio8zSenior Moderator
Total posts : 247
Classically, the function of a choke coil connected from the antenna output of a transmitter to chassis ground is to prevent an electrical hazard in case the output coupling capacitor shorts and causes the B+ of the final to appear on the antenna. Its function is to shunt the DC voltage to ground while allowing the RF signal to pass and it is not intended to protect the transmitter from lightning or static sparks..
The key point here is that a coil can prevent damage if DC or low frequency energy is present on the antenna, but it is not effective for high frequency RF energy which is found in lightning and static spark discharges. I would not waste any time on this.
Use a spark gap arrestor selected for the maximum signal peak voltage expected across the transmitter output. My 100 mW AM transmitter has a 75 Volt arrestor connected before the antenna loading coil to ground (a low impedance point unlike the antenna itself where higher voltages are expected) and has survived for years of outdoor use. You can get them from Mouser or other suppliers for about a dollar each.
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