Patrick, Yes, of course. That's why I cautioned to make sure the power handling of the coax was up to snuff. There is an absolute power limit at each operating frequency unique to any particular coax. It's hotter closer to the transmitting end.
All we have to do is select a coax that can handle 50 watts @ 144 MHz and we're good to go. It's on the data sheet for the coax in question. For instance, the 150 MHz (close enough) loss for Times LMR-100 is 8.9 dB for every 100 feet and it can handle 100 watts average (long time constant - 10 minutes?) power. So, there is plenty of power (heat) margin. If one wants 4.0 dB loss then simply roll out (4.0/8.9 x 100) = 45 feet and put connectors on it. 45 feet of LMR-100 is safe and effective, if a little too long for convenience. Choosing somewhat lossier coax would lead to needing less of it. RG-178 is a Teflon dielectric coax that's very thin and has about twice the loss of LMR-100 and can handle even more power. One would only need 22-25 feet of it for 4.0 dB @ 2 meters. A little web browsing can be marvelous. Cheers, Mike - W8MM BTW, we used to make precision terminations for 450 MHz testing by using a 100 ft roll of rg-174 (with nothing at all connected to the far end). The return loss was twice the attenuation, so the reflection from the open or short at the far end was much weaker than the imperfections of the coax characteristics and connector. Easier, cheaper, and more robust than a precision load. -----Original Message----- From: flexradio-boun...@flex-radio.biz [mailto:flexradio-boun...@flex-radio.biz] On Behalf Of Patrick Greenlee Sent: Monday, August 13, 2012 9:34 AM To: flexradio@flex-radio.biz Subject: Re: [Flexradio] Flex 5000 VU module Wouldn't you want to be careful regarding how lossy the coax is? Too lossy and you have to shed all the watts of energy in a short length of coax so heating might be a problem where a less lossy but longer coax/attenuator would distribute the heat over a larger surface area and not get so hot. Patrick -------------------------------------------------- From: "Mike Valentine" <m...@valentine1.com> Sent: Sunday, August 12, 2012 9:15 PM To: <flexradio@flex-radio.biz> Subject: Re: [Flexradio] Flex 5000 VU module > Why not work around the problem by making an attenuator to knock the 52 > watts down to the 20 watts you need? > > A cheap piece of RG-58 or smaller (lossier) coax of the right length > attached to the input of your amp is a guaranteed way to make it work > right. > > You want a 50/20 = 2.5:1 attenuator which is darn near exactly 4 dB. Wind > off 4-dB-loss of coax and put it between the T/R relay and the input of > your amplifier. You can use your watt meter to prove you've done it > right and test the power output before you hook it up. > > One caution, make sure the coax can handle 50 watts @ the frequency of > interest. > > Bonus advantage: The 4 dB pad will make the amplifier think it's in a > pure 50 ohm system instead of looking askance at the output impedance of > the VU5K. Potential instability problems from things popping outside the > Smith Chart circle are magically pulled back inside ;-) > > I just love pads! > > 73, > > Mike - W8MM > > _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kc.flexradio.com/ Homepage: http://www.flexradio.com/
