I recently purchased an MFJ 225 two port handheld mini VNA for $269.
http://www.mfjenterprises.com/Product.php?productid=MFJ-225
I do not own more expensive gear to test its calibration against, but so
far it appears accurate both for measuring Xl and Xc, SWR and also doing
frequency sweeps for testing filters.
I'd vowed not to buy another MFJ product after my 259B turned out to have a
faulty tuning capacitor, but this one seems to have improved build quality
and uses encoders behind the knobs.
The unit is compatible with the IG-miniVNA software which runs under
Windows. At some point once I have a chance to verify the unit's
calibration I will post a review. My needs at this time are mostly to learn
how to use a two-port VNA and potentially justify the purchase of a higher
quality unit.
73,
Matt NQ6N
On Mon, Mar 28, 2016 at 11:14 AM, Jim Brown <jim@audiosystemsgroup.com>
wrote:
> On Mon,3/28/2016 4:37 AM, KE1F Lou wrote:
>
>> Is the lowest SWR is the resonant frequency?
>> Resonant frequency is when Xl=Xc.
>> When Xl=Xc is not necessarily the lowest SWR.
>>
>
> Exactly right. If, for example, the antenna's feedpoint impedance at
> resonance is different from the feedline impedance, or if the analyzer
> impedance does not match the feedline impedance, the lowest MEASURED SWR
> will be different from the resonant frequency. This happens with lots of
> antennas, including most dipoles, and with most ground-mounted verticals.
> For example, this causes the minimum SWRof my 80 dipole,as measured in the
> shack by a 50 ohm analyzer, to be about 100 kHz below its actual resonant
> frequency.
>
> This is why swept impedance measurements made by an instrument that can
> produce a file that can be transformed by "subtracting out the feedline" is
> so important.
>
> The feedpoint Z of my high dipoles for 80 and 40 is in the range of 85
> ohms, so I feed them with RG11. This minimizes the ACTUAL SWR in the
> feedline (because SWR is determined by the LOAD, NOT the match to the
> transmitter), so it minimizes feedline loss. It does, of course, require
> matching to the rig or power amp in the shack so that the output stage is
> happy, and to minimize distortion in the output stage.
>
> 73, Jim K9YC
>
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