On 07/ 4/11 11:30 PM, Fuqua, Bill L wrote:
> Yes and No.
> You can. to a limited extent, make an antenna system resonate with an
> amplifier with a PI network output.
No, you can't make an antenna resonate. In general an antenna has a complex
impedance R + j X. Only when X == 0 is there resonance.
> Only if it is slightly off resonance the "Load" capacitor and be adjusted
> such that you can get an 'conjugate" impedance match.
> Making the antenna, transmission line and the PI network all resonate at the
> operating frequency and also get maximum power transfer.
I would not consider you are resonating the antenna in this case. You are
getting a conjugate match, which should give maximum power transfer. That's not
the same as resonating the antenna.
> However, this range is limited and the SWR meter will not indicate 1:1 SWR if
> you do get a match becuase the SWR meter is still comparing
> the antenna system to 50 Ohms and also now the reflections in the
> transmission line become part of the "resonante system"
Yes, but 50 Ohms has nothing to do with resonance. An antenna can have a pure
resistive impedance of exactly 50000 Ohms (zero reactance). It has a 1000:1
VSWR
but is resonate.
> Years ago before hams used SWR meters they tuned their transmitters to the
> antenna systems by dipping the plate current and
> adjusting the Loading capactior of the PI network until they got the proper
> correct plate current at the dip. But when using
> linear amplifiers and particularly GG amplifiers this approach does not
> always work out right.
> PI-L networks allow for very little range of compensation for antenna
> missmatch. Still the SWR meter will not indicate a match even
> if you achieved one.
Because you have not achieved a 50 Ohms load.
Since an SWR meter is designed for a 50 Ohm system, then the load must be a
resistive impedance of 50 Ohms to get 1:1 VSWR. In fact, one does not need the
system to be resonate to get a 1:1 readomg, as a 50 Ohm dummy load gives you a
1:1 VSWR, but is not resonate. .
You can have resonance and a VSWR of 1000:1. You can't tell with a load is
resonate by looking at a VSWR meter. A VSWR higher than 1 just indicates its
not
50 Ohms - it could still be resonate. A 50 Ohm load will give 1:1, but that is
not resonate.
The only way to measure resonance is with a vector network analyser or similer
took which can look at the reactive component of the impedance. Only when
that's
zero is there resonance.
> 73
> Bill wa4lav
Dave, G8WRB.
> ________________________________________
> From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf Of
> Dr. David Kirkby [david.kirkby@onetel.net]
> Sent: Monday, July 04, 2011 5:20 PM
> To: 'AMPS'
> Subject: Re: [Amps] High SWR
>
> On 04/23/09 04:50 PM, Scott McGrath wrote:
>> SWR is an obsession with the CB set and some others CB's could not
>> tolerate a high SWR due to the cheap finals used the high RF voltages
>> would blow the junction. This effect created the oral tradition of
>> 1:1 SWR == Antenna System Goodness (tm). It's hard to counteract what
>> 'everybody' knows.
>>
>> In reality and especially with a tube amplifier and a Pi output tank the
>> antenna system can be brought into resonance at a wide range of
>> impedances and resonance is what we are looking for and it really is
>> Antenna System Goodness in MOST cases.
>
> You can't change the resonate frequency of an antenna by adjusting the
> amplifier. The frequency of resonance of an antenna is for all practical
> purposes independent of how the amp is tuned.
>
> You will get 2nd order effects like the RF heating the elements which makes
> them
> expand so changes the frequency of resonance, but I'm ignoring such things
> here.
>
> --
> A: Because it messes up the order in which people normally read text.
> Q: Why is top-posting such a bad thing?
> A: Top-posting.
> Q: What is the most annoying thing in e-mail?
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A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?
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