[UK-CONTEST] Output power

[Chris G3SJJ]

Yeah but an s-point is one helluva a lot, specially on weak signals!!  Just been trying to work 5N7M on 160m and I could do with that s-point. OK and 
SM getting through but not me!

Thanks for the heads up, I couldn't be bothered to work it out

Cheers Chris SJJ



On 31/01/2012 22:00, Steve Knowles wrote:
> 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
>>>
>>>
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>>>
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