Hot matching transformer

[AG5UM]

My antenna "broad-band matching transformer" is overheating, gets hot and SWR shoots up after
about 15 min. of long-wind AM QSO's. It seems to work fine untill it gets hot. It seems to be underrated.
Is it a matter of transformer design, core materials, core saturation,eddy currents ,losses,etc.
Or is a matching transformer just not really a good method for end-fed antennas?
This one is using the dark grey-black core material( two glued together for thickness), and the antenna wire (small awg.)
is just loosely wrapped around the torroid core.
In industry we used all types of special core materials and designs for high-power, high freq. but in
Ham radio it seems simple designs usually do work, anyway,
Do I need a better transformer or is an antenna matching transformer just a high loss- bad idea altogether?
I'm only running about 120 watts AM (Bird meter "average" watts), the transformer is supposed to be...
rated for 800 watts PEP, ICAS-50% transmitt time SSB, So maybe I'm lucky it lasts 15 min....
AG5UM

[AG5UM]

The antenna (with matching transformer) is the MFJ-1982-HP, ("800 watt PEP"),which is probably fine,
for 100 watt "PEP" SSB short transmissions, but does'nt  hold up to my DX-100 AM QSO's.
The Heathkit DX-100 has a Pi-network output that is supposed to tune 50-600 ohms,
I probably don't need "matching transformers" at all.
AG5UM

[WBear2GCR]

What are you "matching"??

Makes a difference.

But if you are going to run one at that power level, better buy some cores
and wind your own.


             _-_-

[AG5UM]

An antenna "matching transformer" matches impedance, say 50 ohms to 600 ohms to reduce the SWR.
AG5UM

[AG5UM]

I guess to be clear, I should have said: impedance matching transformer, in this particular unit it
is configured as a "Unun" (not a balun) and is actually an "auto-transformer", tapped winding.
One end is connected to ground. you can put an ohmmeter on the antenna wire all the way to the
ground of the input coax connector, (zero-ohms).
At least its an interesting design,( made for un-balanced to un-balanced ).
Anyway, its under-rated for my application- it gets HOT....
73
AG5UM

[KK4YY]

If the balun gets hot, that's where a good portion of your power is going. Even if you get a bigger balun, your power will still be wasted in it. The problem isn't the balun, it's the end-fed antenna. Feeding an end-fed antenna is like trying to push on a rope.

[WD5JKO]

Case in point, back when the 20m Monday night AM group was in full steam about 5 years ago, one of my antennas was a full wave vertical loop antenna for 15m. Fed with about 30' of OWL. With a EFJ 275 watt matchbox, I could match that antenna on 20m well, and with a little added external inductor, I could match that antenna on 40m. Anyway, one day I decided to hook up my nice Heathkit 2060A turner instead of the EFJ matchbox. The Heathkit has a nice balun inside, and sure enough I could tune the bands 15-20-40 no problem when using that balun to feed the loop. I get into an AM QSO on 40m with only 40 watt carrier. After a while, the SWR started climbing, so I retuned. After a while, I smelled something, and then there was smoke. My balun was hotter then blazes. Lesson learned, don't use a balun when the load impedance is at an extreme (relative to 50 or 200 ohms) along with a large reactive component. Back to the EFJ!

Jim
Wd5JKO

[Steve - K4HX]

I'd skip the transformer and just use an appropriately sized tuner. Adding or subtracting some antenna length may help too.

[AG5UM]

Thanks for the replies,
This particular antenna was supposed to be a : no-tuner, multi-band, easy put up, simple antenna.
It does seem to be , but not for long AM at my power level.
It does'nt overheat on short transmissions , it takes awhile to heat up.
I liked it just for local, close (100 mile etc.), for simple portable set-ups.
But as was said,
The power is going into the Unun and it seems to be very inefficient,and problematic so far, for my application.
The DX-100 pi-net can tune 50-600 ohms by itself.
End-fed's.... Inverted L, Verticals, etc. work good, I wonder if a nice set of radials would help this antenna?
From your comments it seems I need to give up on it, and maybe on unun-balun's also for the stuff I'm doing.
It seems your saying building a bigger Unun won't help,
It is nice to have a simple-no tuner needed, multi-band simple antenna to toss-up when needed, But not if I can't use it.
Thanks for all your help, and replies,
AG5UM

[AG5UM]

One Good thing about this Autotransformer UnUn is that it keeps the static charges drained off the antenna.
At least thats something.
AG5UM

[Steve - K4HX]

I'm sure that the antenna won't need a tuner because of the loss in the transformer. Any loss will smooth out and lower the SWR curve. AM, even at moderate power levels will reveal lossy components in your RF signal chain. Radials should help your efficiency but won't necessarily make the antenna easier to tune.

Go ahead and connect directly to the DX-100. If you can tune and load to the desired plate current, and (this is an important and) get no arcing under full modulation, you are good to go. One caveat, some 100 watt AM transmitters use a fixed loading caps in addition to the variable cap. In some cases, these can fail with time when tuned into antenna loads outside the specified range. I can't remember what the DX-100 has for loading caps.

[kb3ouk]

Which version of the DX-100 is it? The original had fixed loading caps with a variable, the B used just a variable.

[AG5UM]

I have the original, first model DX-100, not the "B" model, it does have both fixed and variable caps.
It has performed great for several years now. (even with my mistakes)
I've had the antenna quite awhile also, and the DX-100 easily tunes up on it.
It was only when I started using it for long-wind AM QSO's, that it overheated.
The DX-100 manual has an antenna section, for un-balanced wire hook ups, and a design for a
Antenna Coupler-balun for Balanced connections, etc. Some good information.
I realize the UnUn in this antenna is lossy, but it seemed to work, I was wondering what I could do to keep it
from overheating, since it works on short transmissions,
Thanks for the responses and help,
Donnie,
AG5UM

[WBear2GCR]

I guess to be clear, I should have said: impedance matching transformer, in this particular unit it
is configured as a "Unun" (not a balun) and is actually an "auto-transformer", tapped winding.
One end is connected to ground. you can put an ohmmeter on the antenna wire all the way to the
ground of the input coax connector, (zero-ohms).
At least its an interesting design,( made for un-balanced to un-balanced ).
Anyway, its under-rated for my application- it gets HOT....
73
AG5UM

Sorry, totally not clear - the question was what is actually being matched.
Not how or what does the transformer do.

                  _-_-

[Opcom]

Can you describe the antenna or are there plans or a model # for it? Knowing that would be important to know why the balun gets hot, which is almost certainly the cause of the VSWR changing.

[Tim WA1HnyLR]

Jeeez,
I have been trying to tell people for years that the usual hammy hambone Mo Fo Junque and othe high pass :TEE network tuners are JUNQUE . the only case that they will work ok is when you are dealing with an impedance that is with in the design range of the balun,and has little capacitive or inductive reactance. The place for the balun is BEFORE the matching network .IE; The Palstar double balanced "TEE" tuners . The Johnson Match box does work very well but it too does not like dealing with reactive loads. I have found,if the load is capacitively reactive an inductor of the correct value across the antenna terminals will in most cases allow for a great match.
The same is true if the antenna load is very inductively reactive.
 It is time for all of you to roll your own. A pine board ,A couple of air variable condenshers of a suitable rating,and a home made inductor wound on a piece of 2-1/2 " PVC pipe. You can buy a 25 foot length on# 12-2 romex with ground. That is all you need. Ohhh, You must have some clip leads for accessing the taps. The tuner may be easily configured for series of parallel tuning.
Tim WA1HnyLR

[K1JJ]

Yep, as Tron said above, it may be time to build your own tuner:

General instructions:
http://amfone.net/Amforum/index.php?topic=41369.0


Schematics for parallel and series tuning:
http://www.amwindow.org/tech/htm/160smallants.htm


This will put you into a new world of freedom of choice rather than a hostage to MFJ...     By using series or parallel tuning, you can match ANY impedance with very little heat with good system balance.


T

[AG5UM]

Just wanted to say Thanks, to everyone who answered my post. Many great comments, helpful.
I especially wanted to put out a Thank you to:
Jim, w5jo, who's help is greatly appreciated.
Richard Stubbs of MFJ, and Palomar Engineers, who have helped increase my understanding of how to make end-feds work,
and specifically this style design.
I'm in the middle of putting up new poles for a new Dipole and this will give me more options to play with endfeds,etc. also.
I really liked the comments, and as you've said, I think its time to "Roll-my-own" tuner...
Anyway, Thanks,
AG5UM

[DMOD]

I realize i am late to the party but if you are referring to an Unun for impedance transformations here is a schematic and a picture of the one I use for my 43' vertical (untrapped).

This Unun is a homebrew with two stacked cores of 12 Turns bifilar #12 insulated wire.

i used two different colors of wire to keep track of each inductor winding.

Useful to 2kW carrier.


Phil

[WBear2GCR]

Still don't know what the ACTUAL ANTENNA consists of!!

Hard to make good suggestions without knowing what is being, or trying to be matched...
<sigh>

                   _-_-

[DMOD]

Still don't know what the ACTUAL ANTENNA consists of!!

Hard to make good suggestions without knowing what is being, or trying to be matched...
<sigh>

                   _-_-

It would be helpful to know the specifics.

The only thing we know is:

Or is a matching transformer just not really a good method for end-fed antennas?

We don't know if it is a horizontal "end-fed" or if it is actually a vertical "base-fed" antenna.

We also don't know what the measured impedance is at various frequencies.

Phil

[Jim, W5JO]

Still don't know what the ACTUAL ANTENNA consists of!!

Hard to make good suggestions without knowing what is being, or trying to be matched...
<sigh>

                   _-_-

MFJ-1982-HP
https://mfjenterprises.com/products/mfj-1982hp

[DMOD]

Holy Mohly, no wonder that sucker gets hot and is limited to about 150 Watts carrier.

Matching Network Vents

The impedance ratio of the EFHW matching network is very high (49:1), so there
will be some inherent loss. As a result, some of your transmitted power will be
dissipated as heat in the transformer. While there's not enough loss to noticeably
detract from your signal strength, it will cause the transformer's ferrite core to
warm during transmissions -- especially at high power levels. To ensure
adequate cooling, always mount the matching network in the clear and
periodically inspect the screened vents, brushing away any accumulated debris
from the openings

Phil