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   Home   Help Search  
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Author Topic: Balun gets too hot?  (Read 15117 times)

Posts: 47

« Reply #60 on: July 15, 2009, 07:06:45 PM »

>> - Determine the resonant frequency of your folded
>> dipole and assist with tuning it to the correct length.
>> - Determine the impedance of your antenna
>> - Create the 1/4 wave balun
>> - Or even test your toroid by substituting the
>> antenna with 300Ohms worth of resistors.

I have the MFJ-259B.  As it currently is, with the different core I picked, the antenna seems to be resonant 1:1 VSWR at approx 29 Mhz.  Its 1.5:1 or less across the 28.300-28.600 portion of the band.  At 28.400 the impedance is 36 ohms according to MFJ-259.

Looks like I overestimated the twinlead velosity factor and cut it too short.  Or maybe because its not in free space.  I'd have to hang it up between trees or something to see if that makes a difference.

But it works, the rig loads up and I made good contacts within single Sporadic E skip with it.  I haven't done much if at all better with a vertical, operating mobile - of course there I'm only running 25W.

I plan to try a coax bazooka next and cut it a little to long for 0.66 velosity factor at first just in case.  Than we will see how that goes.

I could also try a dummy resistor across the balun to see what my impedance really transforms to.

Posts: 47

« Reply #61 on: July 15, 2009, 07:24:19 PM »

>> Create the 1/4 wave balun

Regarding the 1/4 wave balun, I tried it back last February with almost the same twinlead dipole and ended up wasting a whole lot of RG-58.  Either I wasn't paying attention, or something but I remember I started long and never got anything close to a workable VSWR.  I cut it inch by inch until it became obvious I wasn't going to get anywhere.  Now that I think of it, maybe I forgot to apply the velocity factor to the twinlead dipole in the 1st place and was trying to match a dipole which wasn't even tuned to where I wanted to be to begin with.

I'll try again if I have any RG-58 left over after my bazooka experiment.

The last time I had a "successful" 1/4 wave balun building experience was a long time ago in a galaxy far far away when I lived in the former Soviet Union and tried to match a folded dipole to 75 ohm coax for TV reception.  It worked to my satisfaction, but I never had an antenna analyzer back then and I guess I don't know how well that antenna was matched after all Smiley

Posts: 95

« Reply #62 on: July 16, 2009, 05:31:27 AM »

A twin-lead folded dipole installed at typical residential mounting heights will usually exhibit lowest SWR with a 6:1 impedance ratio transformer or autotransformer (75x4)/6 = 50. Also, it doesn't hurt to install a 1:1 50-Ohm current choke in tandem with the transformer on the coax (input) side.

As for power loss, a well-designed transformer wound on #43 core material should have a measured efficiency of 98% or more at HF, so there's no need to lose sleep over signal loss. However, the transformer's ferrite core will likely get warm with high power applied (2% loss at 1500-W key down still means 30 watts dissipated as heat). Use a core with enough mass and surface area to handle some heating -- and vent the center block enclosure at the bottom to allow for some air circulation.

Posts: 1209


« Reply #63 on: July 17, 2009, 11:19:20 AM »

I would suspect the core described are way too small for 100W @ HF freqs regardless of material type. Sounds like the OP was sold a 'bills of goods' by someone who saw him coming a long ways off.

I would recommend that the OP get some real data on baluns, obtain the proper materials and give it another go.

Gene W5DQ
Ridgecrest, CA - DM15dp

Posts: 805

« Reply #64 on: July 17, 2009, 01:03:07 PM »

This reference suggests that it is indeed the case that someone used an entirely inappropriate core.

It got a little scrambled in copying, but you can see the yellow/white coding designates essentially DC to 1MHz applications. You can see it entire at:

Wouldn't be surprised if the maker just figured all of the same dimensions are alike and modeled it on a published design for winding and rating without attending to the class of core. I would sure expect it to heat up if it is what the yellow/white coding suggests. I guess I'm surprised it didn't heat up more and melt the solder joints. Bet it would under 1 kW.  

Magnetic Properties of Iron Powder Cores

                                      Stability   frequency
Mix Color        Material           u (ppm/deg C)    (MHz)    Notes
--- ------------ ----------------  -- ----------- ----------- -----------------
 26 Yellow/White Hydrogen reduced  75     825     dc - 1      Used for EMI filters and dc chokes
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