Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: MFJ-259C  (Read 1861 times)
KK6YDA
Member

Posts: 65




Ignore
« on: October 08, 2017, 07:50:20 PM »

Never used this analyzer before and it will be arriving next week with my new Vertical. I did some reading on the web about using it and a little confused at some of the answers. I have some band radials to tune before putting it up in it's final position. I am confused using the 259c, will I be looking at the SWR for the lowest with in the frequency range I want, or am I trying to achieve the best Rs (impedance)? or a combo of both using RG-8x 50ohm feed line?
Logged
AC2EU
Member

Posts: 1206


WWW

Ignore
« Reply #1 on: October 08, 2017, 08:16:44 PM »

for a 50 ohm system you ideally want R= 50 ohms, X = 0 ohms, SWR = 1:1
However, antennas are NEVER ideal.

You should read up on "complex impedance" to get the most use from the instrument.
Logged

WB6BYU
Member

Posts: 17061




Ignore
« Reply #2 on: October 08, 2017, 10:23:48 PM »

That really depends on what you need to do to tune the radials.

In most cases, simply adjusting for minimum SWR at your favorite
frequency on each band is probably, and most manufacturers won't
assume that the average ham can measure anything other than SWR
anyway.

One advantage of measuring SWR is that (unless you have common
mode currents) you can measure it through a random length of
50 ohm coax and still get the same result (that is, still minimize the
SWR, even if it isn't exactly the same as at the other end.)

By contrast, most other things you can measure, including R and X,
will vary along the coax, except when it is perfectly matched.  If
you try to take such measurements without correcting for the length
of the transmission line, you can end up adjusting it the wrong
direction, or mess up the settings when it is already matched as
well as it can be.
Logged
G4AON
Member

Posts: 1021




Ignore
« Reply #3 on: October 09, 2017, 12:17:59 AM »

If you are getting a multi-band elevated vertical, you will find interaction between the radials, the better analysers show the SWR on several bands on the same screen, making adjustments much quicker than using the old 259.

Unless you are matching low band verticals, or yagis with hairpin matches, where you look at feed impedance first, then add and adjust the hairpin for 50 Ohms, etc. Then just go for min SWR.

73 Dave
Logged
KK6YDA
Member

Posts: 65




Ignore
« Reply #4 on: October 09, 2017, 11:00:26 AM »

From watching videos on how to tune an antenna using the 259c, I am looking to tune the frequency range with the lowest SWR~Rs and Xs I can get. (example 1.1/40'-50's in ohms and close to Zero as possible on Xs.  I hate manuals, love youtube.. lol
Logged
WB6BYU
Member

Posts: 17061




Ignore
« Reply #5 on: October 09, 2017, 04:06:15 PM »

The SWR is a summary number that tells you how close to R = 50 and X = 0 you are.
That's handy when both numbers shift and you have to figure out whether the net
effect is an improvement or not.

When the SWR is low, the impedance MUST be close to R = 50 and X = 0.  That's
what matters.
Logged
VK2TIL
Member

Posts: 537




Ignore
« Reply #6 on: October 09, 2017, 04:40:40 PM »

A useful accessory for any impedance-measuring instrument is a "sanity check" or, more elegantly, a "precision mismatch"; more than one can be made.

They can be built on the back of a suitable RF connector; the MFJ has an SO-239 so a PL-259 could be used.  An alternative is to use an SO-239 and a m-m adaptor.

Just a resistor will do for a start, soldered between the connector's centre-pin and its body; any value within sensible limits will do.  For example, 100 ohms should read 2:1 SWR; there will be little or no X at the lower end of HF and, perhaps, a little at the higher end due to stray L & C.

Introducing a small series L or C with a resistor can be educational; you then have a series connection of the kind in which Z = R +/- jX.  You can calculate the value of X from the L or C value at a few frequencies and check the analyser this way.

Logged
KK6YDA
Member

Posts: 65




Ignore
« Reply #7 on: October 09, 2017, 07:49:03 PM »

Question in regards to tuning the radials on the antenna, or any antenna for that matter. Do you use the feed line to the antenna you are going to use, or can you just use a short piece of coax to check the antenna with the MFJ-259c and tune the antenna? or does it matter?
Logged
AC2EU
Member

Posts: 1206


WWW

Ignore
« Reply #8 on: October 10, 2017, 07:55:37 AM »

Question in regards to tuning the radials on the antenna, or any antenna for that matter. Do you use the feed line to the antenna you are going to use, or can you just use a short piece of coax to check the antenna with the MFJ-259c and tune the antenna? or does it matter?

Since there will be an inevitable mismatch between the antenna impedance  and the coax impedance, the  impedance seen at the shack end will vary with length.
Test at the shack end. After all, that's where the "rubber meets the road"  Grin
( simplest answer)


If you want to get a rough idea of what the actual impedance of the antenna is at some frequency you can use half wave multiple lengths of coax ( calculated for VF) at the  frequency of interest. Using a half wave length coax might be handy rather than going back and forth between the shack and the antenna .

More sophisticated VNAs will allow you to calibrate for cable effects.

However, all of this is probably well beyond the scope of what you are trying to do, which is to simply get the best match possible for your setup.
Logged

KK6YDA
Member

Posts: 65




Ignore
« Reply #9 on: October 10, 2017, 08:12:06 AM »

I just need to tune my vertical radials before mounting it up on the mast for best performance. I will have an antenna tuner in the shack.
Logged
WB6BYU
Member

Posts: 17061




Ignore
« Reply #10 on: October 10, 2017, 12:40:49 PM »

Quote from: KK6YDA

I just need to tune my vertical radials before mounting it up on the mast for best performance...




Well, one problem is that most hams aren't used to the phrase "tuning radials", so may not know
exactly what this entails.

I pulled up the manual for the MFJ-2389 to see what it actually said, and to see what the
antenna looks like.  Basically it is just adjusting the elements for minimum SWR at the desired
frequency, but in practice it may be more complicated than that.

The tuning steps for the HF portion are basically:

1) Tune the radiator tuning stubs for minimum SWR on each band.
2) Tune the radial tuning stubs for minimum SWR on each band.
3) Repeat 1 and 2 as needed to get a low SWR.

So the only tuning required is for minimum SWR on each band, and you could, in theory, do it
all with just an SWR meter. 

But this is a very narrow bandwidth antenna.  Yes, you can use the tuner in the shack for
fine touch-ups, but any change in the antenna position, or even having your body anywhere
near the radials, can shift the tuning.  And I'd expect a lot of issues with common mode
current that can really mess up your tuning if you don't have an effective balun.

So you can't really adjust the radials before you set it up, other than using the
nominal dimensions in the assembly manual, because raising it to its final position will
affect the tuning.  So plan to put it up and take it down a few times, or use a ladder to
reach it to adjust the radials, then climb down away from the antenna (and remove the
ladder if it is metal) to check the setting.  Having the analyzer on the roof with you will
mean you don't have to back to the shack to recheck the SWR each time.

In this application, an SWR sweep across the band should show the point of lowest SWR,
and you can lengthen / shorten the corresponding radial accordingly to shift that to where
you want it.  If you run out of adjustment room, you'll need to readjust the radiator
stub and try again.


However, the effectiveness of the balun will also make a difference, and a poor balun
(or other coupling between the antenna and the coax) can cause a shift in SWR between
what you measure right at the feedpoint and what you measure at the end of the coax
in the shack.  Otherwise you can use a short jumper to connect the analyzer to the
feedpoint.  I'd probably use a short jumper with the balun/choke(s) that you are going
to use in the final application, and assume that if the resonance shifted I had more work
to do decoupling the antenna.


So in the end I'd get it as close as you reasonably can using the analyzer connected with
a short jumper (and balun/choke) to the feedpoint, then just use the tuner and make it
work.
Logged
KK6YDA
Member

Posts: 65




Ignore
« Reply #11 on: October 10, 2017, 06:11:08 PM »

I'm getting tired just reading the steps, lol... Thank you for the input. Just waiting for the analyzer to arrive.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!