J
Jeff Liebermann
Guest
On Sun, 28 Sep 2014 18:10:42 +0300, upsidedown@downunder.com wrote:
Yep. I accidentally did the same thing using coax for a dummy load
for a 900 MHz paging transmitter. The outputs of the 8 power amps at
125 watts each, are combined to produce up to 1000 watts. The site
was licensed for 4 frequencies, so the power was changing between 125
and 500 watts output.
<http://imgs.inkfrog.com/pix/erac1/Glenayre_Nucleus_011.JPG>
The round thing in the middle of the rack is a typical combiner. I
only had a 100 watt dummy load handy, so I figured that I could use
the coax cable to do most of the attenuation. Bad idea at that power
level.
As you mentioned, the problem with the coax dummy load was that the
heat is not distributed uniformly along the coax cable, with most of
the dissipation in the first few feet of coax. Because the
transmitter sees 50 ohms, the VSWR protection circuitry allowed full
power. The result was that the first 10 feet or so of RG-58c/u melted
and began smoking after about 1 or 2 minutes. Had I not killed the
power, I suspect the polyethylene dielectric would have caught fire.
The rest of the 500 ft roll looked undamaged, so I just cut off the
first few feet that looked melted and put the roll back into storage.
After a few odd problems with using coax from that roll, I put the
roll on a TDR to see if there were any oddities. Sure enough, the
first 50 ft was ruined, probably by having the center conductor move
off the center axis line of the coax while the dielectric was melting.
Cutting short lengths from the bad section showed that the center
conductor had indeed moved off center.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
I have used a 100 m reel of RG-58 as a dummy load for 1 GHz+ . It does
not matter if the opposite end is open or shorted.
For higher power level, you had to unreel some few meters from the
reel to help heat transfer.
Yep. I accidentally did the same thing using coax for a dummy load
for a 900 MHz paging transmitter. The outputs of the 8 power amps at
125 watts each, are combined to produce up to 1000 watts. The site
was licensed for 4 frequencies, so the power was changing between 125
and 500 watts output.
<http://imgs.inkfrog.com/pix/erac1/Glenayre_Nucleus_011.JPG>
The round thing in the middle of the rack is a typical combiner. I
only had a 100 watt dummy load handy, so I figured that I could use
the coax cable to do most of the attenuation. Bad idea at that power
level.
As you mentioned, the problem with the coax dummy load was that the
heat is not distributed uniformly along the coax cable, with most of
the dissipation in the first few feet of coax. Because the
transmitter sees 50 ohms, the VSWR protection circuitry allowed full
power. The result was that the first 10 feet or so of RG-58c/u melted
and began smoking after about 1 or 2 minutes. Had I not killed the
power, I suspect the polyethylene dielectric would have caught fire.
The rest of the 500 ft roll looked undamaged, so I just cut off the
first few feet that looked melted and put the roll back into storage.
After a few odd problems with using coax from that roll, I put the
roll on a TDR to see if there were any oddities. Sure enough, the
first 50 ft was ruined, probably by having the center conductor move
off the center axis line of the coax while the dielectric was melting.
Cutting short lengths from the bad section showed that the center
conductor had indeed moved off center.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558