[AMPS] Benefit
km1h @ juno.com
km1h@juno.com
Sat, 14 Mar 1998 11:54:57 EST
On Fri, 13 Mar 98 23:14:47 -0600 Jon Ogden <jono@webspun.com>
SNIP
> Also, keep your coax runs as short
>as
>possible and use good quality coax and connectors. Minimize losses as
>
>much as possible. This includes making sure you have a well tuned
>antenna. Bad SWR basically causes losses as every time your RF is
>reflected back and forth through the coax, there is a little
>additional
>loss. An SWR of just 1.5:1 can reduce your power by as much as 4% (4%
>of
>power is reflected back from antenna).
I believe that is erroneous Jon, but I am not sure what you are saying
either...so please bear with me.
Here are the published flat and additional VSWR losses for RG-213 per
100':
Freq Flat Loss Add loss 2:1 VSWR Add for 4:1 VSWR
4 Mhz 0.32dB <0.1dB 0.3dB
15 0.8 0.18 0.7
30 1.2 0.22 0.9
It would then appear that even a 4:1 VSWR at 20M is of no real
consequence for many styles of operating. On 80M I really could care less
about the.3dB when I go from CW to SSB.
Losses are much less with CATV hardline and .5 or .875" Heliax. For
instance .75" CATV has a flat loss of 0.25dB at 30MHz; a 2:1 VSWR would
add < 0.1dB and a 4:1 VSWR would add 0.25dB.
I use all CATV and Heliax line here.
The only feedline I have ever punctured was RG-6 CATV drop cable when
trying to make a LP array work on 75M
RG-213 is rated at 5KV continuous. The amp load capacitor is more apt to
arc first.
I know of many hams that have accidentally operated into the wrong
antenna, they have blown loading coils and traps but never RG-213.
73 Carl KM1H
At 1500 watts that is 60 watts
>
>(some of that 60 watts does get eventually radiated after it is
>reflected
>back from the source, travels back downt the transmission line and out
>to
>the antenna, but there are losses with each trip through the coax).
>An
>SWR of 1.8:1 can result in an 8.2% reflection. As well the high
>voltage
>peaks in a condition of high SWR along the coax can cause problems if
>your coax isn't high enough quality (dielectric can literally break
>down). Remember that if you have 1.5 dB of total loss in all of your
>feedline, connectors, etc. you can make your 1500 watts into 1000 at
>the
>antenna! You've also done a lot to heat up the coax!
>
>To anyone buying an amplifier, I would urge them to think in terms
>other
>than just transmitted power. It's best to think in terms of ERP
>(Effective Radiated Power). Again, what can you do to improve your
>station over just adding more power. What can you do with the
>antenna,
>the feedline, etc. It all matters quite a bit. It's an entire system
>
>working together and it is only as good as the weakest link.
>
>In the final analysis however, after all is said and done, I would buy
>an
>amp that is rated continous duty at 1500 watts. If you are going to
>spend the money you might as well do it right. That way you know that
>if
>you run 1200 or 1300 watts that your amp is well within its limits.
>However as another person put it, the 1000 watt amp the original
>poster
>asked about may not even cut it at 1000 watts but more like 700 watts.
>
>>From 700 to 1500 is a BIG difference.
>
>I apologize for the length of this and if any was too technical for
>anyone out there. I just got on a roll! :-)
>
>Cheers to all,
>
>Jon
>KE9NA
>
>
>-------------------------------------
>Jon Ogden
>KE9NA
>
>http://www.qsl.net/ke9na
>
>
>"A life lived in fear is a life half lived."
>
>
>
>
>--
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>
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