[TenTec] Tuning Amp on 17 meters

[Robert Mcgraw]

What ever happened to using the old fashion Millen Grid Dip Meter or the
Heathkit Tunner Dipper to find resonance?  I can also use my MFJ 259
antenna bridge with a 2 turn loop on the input to find the resonant
frequency.  These do exactly what is needed and thus find the resonant
frequency of a circuit or antenna.  Then I use the MFJ or Autec bridge to
adjust the network for 50 ohms thus matching the Z of the resonant
antenna which is more likely not 50 ohms.  From there regardless of
length, the 50 ohm feed line is happy as well as the transmitter or
amplifier feeding the feed line.

All too many times do we hear of hams trimming the antenna for lowest
SWR.  That simply says they are adjusting the length away from resonance
to create some reactive component either +j or -j by increasing or
decreasing the length to make it look like 50 ohms.  Even after doing so,
they do not have a resonant antenna but they do have a resonant load
which may or may not radiate well.

73
Bob, K4TAX


> Jim,
>
> You are 100% correct but in a case like James has, I think you are kind
> of
> splitting hairs.
>
> "If" the antenna is a monoband and fairly close to being 50 Ohms
> resistive,
> there won't be much transformation due to the transmission line, and the
> resonance frequency measured will be very close to accurate.
>
> Specifically, in the case of the RigExpert analyzers (which James also
> has),
> I set mine to Z=R||+jX, find the two points where the Xp changes
> directions
> and take the median.  This is very close to the true resonance frequency
> of
> the antenna.  It is, as measured on 4 different antennas here, usually
> 100
> to 300 kHz away from the point of minimum SWR.
>
> If I had a significant mismatch, then the transmission line would indeed
> induce a significant transformation, but based on dipoles cut and trimmed
> to
> the theoretical calculated length, (or James' vertical), these aren't
> going
> to be very far off, are they?
>
> My basic point to James was on his monoband vertical, measuring in
> another
> mode would give him a more accurate indicate of resonance than SWR.
>
> Hw?
>
> Rick, DJ0IP
>
> -----Original Message-----
> From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Jim
> Brown
> Sent: Thursday, April 18, 2013 6:13 PM
> To: tentec at contesting.com
> Subject: Re: [TenTec] Tuning Amp on 17 meters
>
> On 4/18/2013 1:51 AM, Rick - DJ0IP / NJ0IP wrote:
>> It is very easy to confirm that with all of your antennas, using your
>> RigExpert AA-230, but NOT in SWR mode.
>> Switch it to Phase or one of the R/X/ modes.  There you can find the
>> true resonance frequency.  Then go find the frequency of minimum SWR.
>> They are almost never the same.
>
> Not that easy unless the impedance of the antenna at resonance is the
> same
> as the impedance of the transmission line, and many (most) antennas are
> not.
> The impedance of the antenna is transformed by the transmission line, and
> both the transformation and the math that predicts it are complex.
>
> To find the resonant frequency, you must first be able to measure the
> complex impedance (that is, R + jX) at some point on the transmission
> line
> (could be in the shack) save that data as a standard interchange file,
> measure the electrical length of the line, export the complex data to
> Smith
> chart software, and rotate the measurement around the Smith chart the
> number
> of electrical degrees corresponding to the length measurement that you
> made.
>
> The good news is that very good Smith chart software is both FREE and
> easy
> to use. it's called SimSmith, runs in Java, and google will find the
> download site.  More good news is that once you have that software you
> can
> use it to design nice matching networks. The bad news is you'll have to
> study how transmission lines work. :)  But more good news is that you'll
> learn stuff, which is a fundamental objective of ham radio.
>
> 73, Jim K9YC
>
>
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