The point is that I think the question was not stated as it should have been,
however, I realize what he really needs to know.
Actually, many of the old transmitters would work just fine with a wide range
of mismatches and thus making the plate tank circuit
part of the antenna system .
There is much more to resonance that just matching capacitive and inductive
reactances.
It is intereting that before vacuum tubes electrical resonance was well
understood. That is where the name "Tank Ciircuit" came from.
It is a tank becasue it is a storage system of oscillating electrical energy.
The energy stored is proportional to the Q of the tank circuit.
Q= Stored energy divided by the energy leaving (loss) the each cycle. It is
also equal to the resonant frequency divided by the
-3dB bandwidth.
73
Bill wa4lav
________________________________________
From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf Of
Fuqua, Bill L [wlfuqu00@uky.edu]
Sent: Monday, July 04, 2011 8:04 PM
To: Dr. David Kirkby; 'AMPS'
Subject: Re: [Amps] High SWR
You can make the antenna SYSTEM resonant. That is what an antenna tuner does.
By matching with making the -JX with a +Jx the system becomes resonant, the
antenna, transmission line and matching system
all together make a resonant system. And you can get just as much radiated
power as you could get into an "Resonant antenna"
as long as you keep the losses in the tuner and transmission line down.
73
Bill wa4lav
________________________________________
From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf Of
Dr. David Kirkby [david.kirkby@onetel.net]
Sent: Monday, July 04, 2011 7:06 PM
To: 'AMPS'
Subject: Re: [Amps] High SWR
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?
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
--
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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