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

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: MFJ-259B --> Toroid transformer  (Read 2594 times)
NA7I
Member

Posts: 9




Ignore
« on: September 01, 2009, 09:22:07 AM »

Here's a puzzlement, at least to me:

New MFJ-259B, works just fine for me, to tune wire antennas, via 50' of 50-ohm coax feeder. (Batteries, OK, diodes OK, etc. Unit appears to be working.)

In shack:

1) I have 2 inches of 50-ohm coax from a PL-259 plug, plugged into antenna port of MFJ-259B. At end of coax is a 50-ohm resistor (shield to center wire of coax). MFJ shows SWR 1:1 to 1:2 between 7 and 25 Mhz, with impedance about 49 +j2. OK, this looks good.

2) Now I connect a T50-2 toroid transformer, with the same 50-ohm resistor across the secondary, primary to 2 inches of coax, to MFJ-259B. On the T50-2 are 5-turn windings each, for both the primary and the secondary.

One would expect 1:1 transformer action, and similar readings to case (1). But no. Readings are SWR>25 at all HF freqs, with Z = 0 +jX, where X is apparently the inductive reactance of the primary alone. Apparently the inductance of the primary is about
0.6 uH. (L = X/w.)

It does not matter whether the secondary is open circuited, shorted, or what resistance is placed across it. I have rewound the secondary in 'opposite' direction: no effect at all. In all cases, I get the same result described above.

If one is going to use a toroid transformer as part of a matching circuit at the antenna terminals, fed thru, say, 50' of coax, how is the MFJ-259B going to help tune the antenna ? It cannot even see the secondary circuit, while on the workbench, only 2 inches away. How will it possibly do better at the business end of the coax feedeer?

Some hams have posted circuits that are used in this way, so I'm curious. What am I missing here?

Thanks for any comments.

Dick Chaffer   NA7I
Bozeman, Montana
Logged
AD5X
Member

Posts: 1432




Ignore
« Reply #1 on: September 01, 2009, 09:40:20 AM »

5T on the T50-2 is only about 0.1uhy.  Even on 10 meters, the inductive reactance is only about 20 ohms or so.

Phil - AD5X
Logged
NA7I
Member

Posts: 9




Ignore
« Reply #2 on: September 01, 2009, 10:21:23 AM »

Thanks Phil; my mistake. It is a T80-2. Even so, the formulas are hardly precise.

I guess the main concern is not the inductance value. The MFJ-259B is working, as can be seen from my post. The point is that, no matter what the secondary circuit, the MFJ reads the same, as described above.

MFJ has not been able to answer the question, either. They suggest shortening leads by eliminating the coax, which hardly addresses the problems anticipated when tuning via 50' or so of coax.

Dick Chaffer   NA7I
Bozeman, MT
Logged
AD5X
Member

Posts: 1432




Ignore
« Reply #3 on: September 01, 2009, 11:00:59 AM »

The inductive reactance needs to be at least 4X the impedance at the lowest frequency - or at least 200 ohms since you are making a 1:1 transformer.  So you'd need about 30 turns for the primary and 30 turns for the secondary of you want this to work on 40 meters.  That's why you normally use high permiability ferrite cores for broadband RF transformers, not powdered iron.

Phil - AD5X
Logged
NA7I
Member

Posts: 9




Ignore
« Reply #4 on: September 01, 2009, 12:20:31 PM »

Phil - I would be very interested in hearing more about this '4X' rule. It seems like one of those rules of thumb that populate our hobby, but without any foundation in reality. What is the theory or experience behind this claim??

In addition, there are a number of hams' websites which show designs and photos of such transformers in use, with only a few turns (2) on the primary. AA5TB comes to mind. I wish I could get Steve to respond - perhaps he's moved on to other pursuits.

If you believe this 4x rule, why? (other than somebody's quote).
Logged
VK1OD
Member

Posts: 1697




Ignore
« Reply #5 on: September 01, 2009, 01:00:39 PM »

Dick,

What you observe is predictable, ie expected, and it is not surprising that the folk at MFJ can help you with basic AC circuit theory.

eHam is not a good channel for explanation of basic AC circuit theory.

I was going to pen you an email, but like so many correspondents on eHam, it is not easy to find your email address.

You haven't given a hint of your application, but you have discovered that the '259B isn't suited to measurement of the thing you made.

Owen
Logged
HFRF
Member

Posts: 1




Ignore
« Reply #6 on: September 01, 2009, 01:06:17 PM »

You don't seem  to understand the significance of the frequency in this experiment you are doing since you don't bother to disclose it.

You don't seem to understand the significance of the core characteristics, number of turns on the toroid, or the effect of the winding reactance of the toroid as a percentage of the the resistance of the resistor.

Yet you blow off someone giving you good info and depand proof.  Why don't you do some work and learn something about RF.  I am not going to spoon feed you answers with your attitude and work ethic.
Logged
NA7I
Member

Posts: 9




Ignore
« Reply #7 on: September 01, 2009, 01:28:43 PM »

"...eham is not a good place to discuss a.c. circuit theory..." (or words to that effect.

This is the only comment any of you have made that is anywhere near the truth. One might expand it to include 'Any' subject discussed on here.

'HFRF' - apparently afraid of real discussion, is transparently incapable of responding to simple questions. There are a boatload of phony 'rules-of-thumb' floating around ham radio - many of them completely unfounded in theory or fact. Please pardon me when I run across some technical point new to me, and have the temerity (oops, big word for you, HFRF) to ask 'WHY ?'.

This is hardly a complex a.c. problem - except possibly to you, HFRF aka first responder.

Sheesh - what a waste of my time.
Logged
WB6BYU
Member

Posts: 13335




Ignore
« Reply #8 on: September 01, 2009, 02:08:10 PM »

While there are many considerations, perhaps the first
one to consider is the difference in the toroid core
type.  Most broadband transformers are wound using a
ferrite core, while the -2 material is powdered iron, which
has very different characteristics.

Whenever you have a link-coupled coil (any sort of
transformer), the impedance of a winding is that seen
through the transformer action shunted by the impedance
of the winding.  The transformer action depends on the
turns ratio and the coefficient of coupling between the
windings.  If the winding has a low inductive reactance
(as is the case here), that will predominate in the
overall impedance.  The 4x rule of thumb says that, when
the inductive reactance is at least 4 times the impedance
across the winding, it will have minimal effect.  While
this is not an exact requirement, the discussion is more
whether it should be 5x or 10x in some circumstances, not
whether shunting 50 ohms with 10 ohms of inductive
reactance will still give a result close to 50 ohms:
it won't.

Yes, there are many instances in QRP rigs that use a
few turns on a -2 core, but these generally have a tuned
winding on the same core.  But still, in designing such
circuits, the 4x rule (or something to that effect) is
commonly applied. You'll also see fewer turns used on
the higher bands, where the same winding will have more
reactance.


Try winding the same transformer on a FT-x-43 core and
see what a big difference it makes.
Logged
K4DPK
Member

Posts: 1077


WWW

Ignore
« Reply #9 on: September 06, 2009, 09:35:40 PM »

A man named Ruthroff, who was a Motorola engineer, is considered to be the father of the broadband transformer.  He set the guideline for having the lowest impedance winding to have an inductive reactance of at least 4X the impedance, at the lowest anticipated operating frequency.

You should read some of his writings in the Motorola
application note library.

Phil C. Sr.
k4dpk
Logged
K4DPK
Member

Posts: 1077


WWW

Ignore
« Reply #10 on: September 06, 2009, 09:48:08 PM »

Helge Granberg also did extensive work with transformers as an applications engineer for Motorola.

Look up his Motorola App No AN-749.

I may have been mistaken about Ruthroff's employer.  He may have been with Bell Labs.

In any case, look up his paper "Some Broad-Band Transformers", which appeared in the August 1959 proceeding of the IRE.

Phil C. Sr.
k4dpk
Logged
WA3SKN
Member

Posts: 5494




Ignore
« Reply #11 on: September 07, 2009, 06:28:13 AM »

The MFJ259b is a wonderful in field test set... but please remember that it's specs limit it to 0-1000 ohms resistive and 0-650 ohms reactive, you can easily exceed these with many components and frequencies.  Also remember that the "SWR" reading is based on 50 ohm reference.  So your SWR greater than 25 just states that the reading is not within the range of the set at the frequency measured.
Just as toroids have range limits, so does test eqpt.
73s.

-Mike.
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!