Re: [FT897] Tuner SWR or Radio SWR
- Remember too, that the swr display on the 897 is APPROXIMATE - NOT ACCURATE! If you want an accurate reading, an external meter is required. I would trust the MFJ over the radio's display. The reading on the radio will get you out of serious trouble, but I always trust the external meter. Also, remember an antenna tuner is more a "coax tuner" since it matches the feedline AND antenna to the radio (well, actually the feedline moreso than the antenna unless you have the meter AT the antenna). Anyway, just wanted to drop my 2 cents worth.
Dennis - N8BMB
From: Jim <liteways@...>
Sent: Monday, February 2, 2009 11:40:32 AM
Subject: Re: [FT897] Tuner SWR or Radio SWR
The SWR represents the match between the feed line and the antenna. A
50 ohm feed line feeding an antenna with a 100 ohm feed point will give
a 2:1 swr. There is no need to measure the swr at the antenna, because
it will be the same at the other end of the feed line.
Jim Webster wrote:
>[Non-text portions of this message have been removed]
> but in reality you must measure the swr at the feed point of the ant.
> in order to know what it really is, (hard to take a reading at the ant
> when it is up in the air). So if you get different readings (tuner vs
> radio) I would go with the 897D as being more acurrate then the MFJ.
> Jim KI6PY
- ----- Original Message -----
>From: Randy Charles FordBack to the original topic then, the mismatch between the two SWR meters.
>Sent: Friday, February 06, 2009 17:18
>Subject: Re: [FT897] Re: Tuner SWR or Radio SWR
>Is there another side of this to consider? Does the phase of the reflected
>signal matter? Depending on the frequency and distance to the mismatch,
>won't the phase of the reflected signal either add or subtract from the
>transmitted one? Isn't consistency in power across frequencies important?
First: in a transmission line the SWR is the same in every point, provided
- the line is lossless
- the line does not radiate
- The impedance is the same along the transmission line
So in a perfect coaxial line the measured SWR is the same in every point.
Now, losses mean that in practice the SWR is the highest at the antenna
feedpoint (assuming the end of the coax feeds a radiator). At the rig the
SWR (read reflected power wrt forward power) is lower because some of the
reflected power is dissipated on the way back.
In de OP’s case line loss cannot be the reason for the mismatch between the
two meters because the SWR is not consistently lower at the MFJ. Besides,
the difference it too big to be caused by line losses (probably less than a
meter of coax at HF frequencies).
Patch between rig and tuner radiating: Possible, if there is no good defined
common HF ground _AND_ the coax between the rig and the MFJ is not too short
compared to the wavelength (> 1/10). Unlikely again.
Impedance problems: most likely cause in my opinion. Possible the patch is
not 50 Ohm, or there are connector problems, impedance bumps along the
rig/patch/tuner/tuner SWR meter _and/or_ inaccurate meter at the MFJ. Often
the SWR meters in these tuners cut some corners to save a few cents.
Use the SWR value at the rig. All meters are very good at detecting 1:1 SWR
and this is what the rig wants to see to output full power. That the SWR is
not 1:1 further down the line is not so important (only for losses).
Pity however that the MFJ sometimes disagrees because this makes it very
difficult to use the autotuner at these frequencies…