[UK-CONTEST] Output power

[Chris G3SJJ]

Exactly Derek, nicely explained.

As I wrote the original comment I will add some more info as it seems the concept of "System Loss" is something many amateurs don't understand.

The "system" here is 100m of W103 from the shack in the house down the paddock to the main mast. At the base of which  is a metal box which houses the 
WX0B SixPak remote relay switch, then two runs, approx 20m each, of RG213 to the 80/160m dipole and A4S beam. In total there are 6 PL259s, plus one 
back to back connector to join the two ends which I sliced through with the mower

I estimate the "system loss" on 160m to be around 1dB absolute minimum, may be more if you reckon the connectors have been installed over 5 years and 
are not particularly well protected. According to the RSGB Radio Data Reference Book Third Edition, complied by George Jessop G6JP, CEng, MIERE, 1 dB 
is a power ratio of 1.26. Perhaps my interpretation is wrong, but I have assumed that to get 400 w to the antenna I would need to drive it with 500w.

The antenna had an indicated SWR of 1.4 on 1.865 and from memory an indicated reflected power of about 50w. I think theory suggested some of that is 
re-reflected, although the temperature indication on the amp didn't suggest so! Down around 1810 the reflected power was quite horrendous.

Of course, 160m is the least lossy band but if you consider my 40m antenna then "system loss" is going to be higher. From the Sixpak there is a 
further run of approx 40m of RG213 to the phasing box, another PL259 / SO239, an L match network and a vacuum relay, two more socket and plug combos 
and the phsing lines to each element of the 2 ele phased array. Assuming a 2db system loss (ie power raio of of 1.58) and driving the array with 632w 
I still doubt whether 200w would be delivered to each element!! (Seems OK though as I worked VP6T this morning at 0930!)

I suspect on 10m the system loss is approaching 3db, and also I have recently noted an SWR of around 1.7 down the CW end so I guess I am almost into QRP.

Perhaps the best thing would be to do some accurate measurements down the far end but I think I might be amazed at just how lossy the system is and 
would then want to invest in new decent coax and hardware!! (Hmm, methinks a summer project!)

73 Chris G3SJJ



On 31/01/2012 17:43, G3RAU at aol.com wrote:
>
> In a message dated 31/01/2012 17:14:17 GMT Standard Time,
> tony at llewellyns.freeserve.co.uk writes:
>
> Can  someone please explain to me as a non HF contester how it is legal to
> be  using  " Acom 1000 at around 600-700w from the amp. " at any time let
> alone during a contest.
>
> Regards
> Tony  G8ONK
>
>
>
> It is the Laws of Physics Tony.  If you want to deliver 400w to the
> antenna end of a feeder you have to put more than 400w in at the drive end,
> unless you use a theoretical zero loss feeder, which obviously doesn't exist.
> To work out just how much extra power you need to put in, you have to know
> the feeder loss, and this is usually measured in dBs.  Let us say the
> feeder loss is 3db, then the power needs to be increased by 3dB.
> It  can be shown mathematically that this means the drive power needs  to
> be 800 Watts.
> It is as easy as that, just increase power to match the feeder loss  and
> achieve 400w at the antenna.
> A 3dB loss is nothing in some situations, so it is all perfectly legit  and
> nothing to worry about.
> 73
> Derek G3RAU
>
>
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