[Top] [All Lists]

Re: [Amps] High SWR

To: 'AMPS' <>
Subject: Re: [Amps] High SWR
From: "Dr. David Kirkby" <>
Date: Tue, 05 Jul 2011 00:06:50 +0100
List-post: <">>
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 
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 
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 
zero is there resonance.

> 73
> Bill wa4lav

Dave, G8WRB.

> ________________________________________
> From: [] On Behalf Of 
> Dr. David Kirkby []
> 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

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

<Prev in Thread] Current Thread [Next in Thread>