[amsat-bb] Re: DC-feeding through Coaxial on PA?

Angel Peláez, EA4DUT angelkilroy at yahoo.es
Wed Feb 28 08:24:43 PST 2007


I tried once with a VHF 25W watts amplifier fed with 10m of RG-213, just for fun, and it did not work fine because the ohmic losses on the coaxial due to the cross section of the inside conductor (small even for an RG-213) made unacceptable the voltage up at the antenna even with a comsumption of barely 3 Amperes at 13.8V in normal conditions.
One solution is to put, say 24 Volts down at the QTH, and then stabilize to 13.8V up at the amplifier, but this is a power waste.

This method works fine with preamps because the current they need is only of few miliamperes so the voltage fall at the preamp is small and the preamp works.

For example, suppose your feed line has 0.5 ohms of DC resistance from the shack to the antenna and your 100W amplifier needs 20 A at 13.8V to work properly. The voltage fall due to this resistance at this current is 10V!!! so only 3.8V will appear and will not suffice to feed the amp. One way is to reduce the ohmic losses (more copper section and therefore more weight, more money....) and the other one is to increase the DC voltage as I told. None of both is economic nor smart.

I suggest you, if it is possible, to put the 220V mains up at a box by the antenna with a power supply and the amplifier inside. The 220V mains must not be directly connected, rather a sort of galvanic transformer is recommended as well as differential protection switch of 30mA.......Too complex, bulky and expensive perhaps!

Perhaps be better to invest on a low loss line with the additional benefit of low RX losses if used for both TX and RX.

I'm sure many others had sorted out this issue cheaper and easier, so any correction to me will be wellcome and I will learn new experiences.

Best 73.

EA4DUT, Angel

----- Mensaje original ----
De: SV1BSX <sv1bsx at yahoo.gr>
Para: Amsat-BULL <amsat-bb at amsat.org>
Enviado: miércoles, 28 de febrero, 2007 16:03:09
Asunto: [amsat-bb] DC-feeding through Coaxial on PA?

Hi all,

just I wonder if is acceptable the power feeding of a mast UHF-linear (ie 100 W behind of Antenna)
through the same Coaxial cable (200 ft) for RF-driving.
Yes I know, it's a common method with mast Pre-Amps etc. But, when the RF-power input-driving of Linear is 
high enough, ie 20 W or more, is it safe & acceptable to feed the DC-power voltage together with
RF-signal on a common Coaxial?  
Ofcourse in the input of Linear I will put the necessary  "DC-block cap" & RF-chokes with additional
components in order to separate high enough the DC-feeding from RF-signal and a High-Power "T-Bias" 
down in the Shack. 
However I wonderering... I don't like the "toasted"  MRF transistors ! 

Anyone has experience with that?

Thanks in advance

73, Mak SV1BSX

sv1bsx at yahoo.gr

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