Thanks Steve. That makes perfect sense. What's a mystery is why the loss
attributed to SWR under that condition shows as a negative value in TLD,
albeit a small amount. The inverse of loss is...well, gain!
Paul, W9AC
----- Original Message -----
From: "Steve Hunt" <steve@karinya.net>
To: <towertalk@contesting.com>
Sent: Tuesday, November 30, 2010 1:24 PM
Subject: Re: [TowerTalk] tuners and power rating
> 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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|