[MFJ-Ameritron] MFJ-259B impedance reading

[Dick KF4NS]

Ok guys, you got me. I should have been more detailed in my post.

It is an RF test termination made my TEK and is enclosed in a BNC 
male/female adapter. There are no leads. I tested it on my bench top 
HP digital meter, reading 51.8 ohms, and on my Fluke DMM, reading 51.7 
ohms. Not exactly 50 ohms but pretty darn close.

Sorry for the lack of detail.

73, Dick KF4NS
St Petersburg, FL 33714 USA
Keep The Glow!

> -----Original Message-----
> From: mfj-ameritron-bounces at mailman.qth.net [mailto:mfj-ameritron-
> bounces at mailman.qth.net] On Behalf Of Dick KF4NS
> Sent: Thursday, June 23, 2011 3:04 PM
> To: mfj-ameritron at mailman.qth.net
> Subject: [MFJ-Ameritron] MFJ-259B impedance reading
>
> This group seems to have been dead for some time but will give it a
> try anyway.
>
> I never tried this before but was messing with my antenna tuner and
> decided to compare my antenna impedance reading with a precision 
> 50ohm
> resistor. I noticed that the resistor reads 58 ohms (much to my
> surprise) when connected directly to the antenna jack on the  259B.
>
> Is my instrument in need of calibration or is this typical? I would
> think pure resistance should read right on.
>
> 73, Dick KF4NS
> St Petersburg, FL 33714 USA
> Keep The Glow!



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Message: 4
Date: Fri, 24 Jun 2011 00:26:07 -0400
From: Andy <ingraham.ma.ultranet at rcn.com>
Subject: Re: [MFJ-Ameritron] MFJ-259B impedance reading
To: mfj-ameritron at mailman.qth.net
Message-ID: <BANLkTi=UN=nu7fJaw8tZ4zYviS_UX==VGA at mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

> I never tried this before but was messing with my antenna tuner and
> decided to compare my antenna impedance reading with a precision 
> 50ohm
> resistor.

What exactly does "precision resistor" mean?

Can you measure its resistance with an accurate DVM?

Is it supposed to be a good RF resistor too?

At what frequency did you measure it on the '259B?

Using what leads, or connectors?  How is it constructed?

> I noticed that the resistor reads 58 ohms (much to my
> surprise) when connected directly to the antenna jack on the ?259B.
>
> Is my instrument in need of calibration or is this typical?

That depends.  See previous questions.

> I would
> think pure resistance should read right on.

Well, "pure resistance" that is known to be exactly 50 ohms, should
measure 50 ohms.  Why do you say it is a pure resistance, and do you
know that it really is 50 ohms?

For example, it might be a good 50.0 ohm resistor, but inductance of
the leads could make the Z measure higher.

Even dummy loads, which are supposed to be good over a wide range of
frequencies, can measure off from what you'd think they ought to be.
I've got a good dummy load that measures quite nicely; and some others
that surprisingly do not.

Andy


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