Paul,
It's not really counter-intuitive when you remember that at HF the vast
majority of feedline losses are I*I*R losses and therefore driven by the
current that is being carried. If we have a 150 Ohm Resistive component
to the load - no matter what its Reactance - the current into the load
must be 57.7% that of the matched case when dissipating the same power.
That means that *right adjacent to the load* the feedline power loss per
unit length will be 1/3 that of the matched case. It's only further back
along the line, where the current standing wave begins to "develop"
higher currents, that the losses increase and eventually overtake the
matched-line losses.
ARRL's TLW software gets it right. Now, if they would just correct their
hopelessly optimistic loss figures for ladderline, it would be a useful
programme ;)
73,
Steve G3TXQ
On 30/11/2010 14:09, Paul Christensen wrote:
> TLD software appears to satisfy that test. What's interesting under your
> test set of parameters is that power delivered to the load peaks when the
> complex Z at the load is opposite in reactance at the Tx end, but the
> resulting Z values are equal. For example, if you use a load of 150 +j130,
> the resulting complex Z seen at the Tx is 150-j130, resulting in 99.7% of
> power reaching the load. The two resulting impedances are exactly equal
> being just under 200 ohms. I am using 10 ft. of RG-213 (vf = 0.66) for the
> simulation.
>
> By contrast, under a matched condition, where the termination is 50+j0,
> power delivered to the load is only 99.17%. Small differences for sure, but
> the result is very interesting and counterintuitive. According to TLD, the
> higher power delivered to the load under the mismatch condition is the
> result of loss attributed solely by SWR actually going negative in value
> (-0.024 dB).
>
> Paul, W9AC
>
> ----- Original Message -----
> From: "Steve Hunt"<steve@karinya.net>
> To:<towertalk@contesting.com>
> Sent: Tuesday, November 30, 2010 7:29 AM
> Subject: Re: [TowerTalk] tuners and power rating
>
>
>> The most comprehensive calculator I know is Owen's at:
>> http://vk1od.net/calc/tl/tllc.php
>>
>> A good test for a calculator is to look at the loss it predicts for 10ft
>> of 50 Ohm coax at 3.5MHz with a 150+j0 load (VSWR=3:1); the loss should
>> be about **half** the matched loss.
>>
>> Charts which imply - without any caveats - that a particular value of
>> VSWR always adds a fixed "excess loss", are misleading and show a lack
>> of understanding of the loss mechanisms.
>>
>> 73,
>> Steve G3TXQ
>>
>> On 30/11/2010 00:04, Rik van Riel wrote:
>>> After some searching around the web, I found an online
>>> transmission line calculator that does take these things
>>> into account:
>>>
>>> http://www.stroobandt.com/antennas/tl-calculator.html
>>>
>>> It does indeed give interesting and different answers
>>> than the coax calculator I used before...
>>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk@contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|