[TowerTalk] Feedline (choke) question

[Steve Harrison]

On 10/3/2025 1:32 PM, Michael Tope wrote:
> On 10/2/2025 11:09 AM, Steve Harrison wrote:
>> On 10/2/2025 10:01 AM, jim.thom jim.thom at telus.net wrote:
>>> If it's hitting  300 deg F..... I would say you are using the wrong 
>>> connector !
>
> Steve,
>
> I wonder if something other than the N-connector was the source of the 
> heat? Perhaps something inside the device with the hot N connector (I 
> think you said it was a combiner) was getting really hot and 
> conducting that heat through the connection to the type-N female pin 
> which in turn conducted it to the male connector.

It was definitely the connector; I was going to bring the following up 
about something Jim said last time but decided not to at the time.

Let's do a power loss calculation; let's assume the connector has just 
0.05 dB loss: antilog (0.05/10) -1 = 0.0116 = 1.116%. 1.116% of 1000 
watts = 11.16 watts, dissipated entirely within that type N connector 
(with some heat convection to things on either side of the connector, of 
course). We might add additional loss of the center pin due to heating 
from the high RF current flowing through it; but I don't have a figure 
for that. And lest you doubt the assumed loss of 0.05 dB, remember that 
these were LABORATORY-usage connectors and cables; thus, they already 
had lots and lots of prior usage with the accompanying wear and tear. 
It's entirely possible the actual power loss was double that, or as much 
as 22 watts. Take a little dinky 22-watt pencil-type soldering iron, and 
hold it to a type N connector for awhile. It won't get quite hot enough 
to melt solder, but it certainly will get hot enough to burn your fingers.

This cable, which was the old-fashioned 1/2" almost-solid Heliax (what 
did they used to call that stuff: FSJ4 or something like that??), was 
about six feet long and ran first through a Bird 43 wattmeter with HN 
connectors, then another hunk ran to a 2500-watt Bird fan-cooled dummy 
load. I was able to buy a type LC connector for the Heliax to mate with 
the Bird load's LC connector. Thus, there were no other such "low-power" 
connectors to heat up.

> That would explain why that same N-connector was NOT getting hot on 
> the old tube transmitter.

It WAS; as I said, the customer's technicians showed me both female 
chassis-mounted and male type N connectors that they had burned up on 
the tube transmitters. This was why they always had two additional 
transmitters ready to immediately take over when (not if) the first one 
quit.

>  The one in the SSPA was acting like a heat sink for something very 
> hot inside the combiner.

It's been 35+ years, but as I recall, I measured that 8-way power 
combiner to have around 0.15 dB loss, outstanding for multiway power 
combiners, especially at UHF. 0.15 dB is about three times the loss of 
the single type N connector, or around 33 watts. The power combiner was 
not only inside the pressurized chassis: it also had cooling air both 
blowing directly on it as well as circulating all around it. It was in 
no danger of overheating. I left the bottom cover off one transmitter 
one time just to check how hot the combiner was getting: it felt like 
room-temperature to me.

Steve, K0XP