[UK-CONTEST] Output power

[Steve Knowles]

Hi Chris

Re your 10m .....

Using the same Reference Book (and utilising the figures given in the 
tables), the worst possible SWR with 3db of cable loss is 3:1 (13 Watts up, 
3.25W back after cable losses, given zero loss at the reflection)
With a feedpoint SWR of 1.7:1, for every 13W you are putting up the feeder, 
1 is coming back.  With 3db of cable loss that means  6.5W are getting to 
the antenna on the upward journey.  For 1W to get back to the measurement 
point, that means just 2W are being returned from the antenna.  So the 
antenna is absorbing 4.5W.  This means the SWR at the antenna is about 2.7:1 
and it is disposing of over 60% of the power it receives;  regrettably 
that's only 30% of what you generate but it's still a long way off QRP!

To put things in perspective, you're only going to be about 1 'S' unit 
weaker than if the system were lossless.  Makes you wonder if all the effort 
we go to is worth it, HI!!

Cheers

Steve  UFY


----- Original Message ----- 
From: "Chris G3SJJ" <g3sjj at btinternet.com>
Cc: <uk-contest at contesting.com>
Sent: Tuesday, January 31, 2012 9:14 PM
Subject: Re: [UK-CONTEST] Output power


> 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
>>
>>
>> _______________________________________________
>> UK-Contest mailing list
>> UK-Contest at contesting.com
>> http://lists.contesting.com/mailman/listinfo/uk-contest
>>
> _______________________________________________
> UK-Contest mailing list
> UK-Contest at contesting.com
> http://lists.contesting.com/mailman/listinfo/uk-contest
>