The AM Forum

Some of you may remember my thread of being off 75 meters.  Well I'm back on 75 with a MFJ 918 which consists of a piece of RG-303 with Teflon dielectric and some ferrite beads along the coax.  It seems to be working.  I am going to have to redo my antenna length as it is too short.  It was originally cut for an inverted VEE which called for a 5% reduction in length. That will be taken care of when the new antenna goes up. The 918 will be replaced with a Palomar Engineering 1:1 5KW balun.  But I digress.

I wanted to test the blown balun and it turns out, I had an extra balun that was good.  All I did was jury-rig a 50 ohm scope feedthrough across the balun leads that would be attached to the dipole.  The good balun was around 1.5:1 across the 160 to 10 meter bands.  The bad balun has varying values but at the test frequency was 6.2:1.

Al,

I'm surprised that you did not choose to "wind ur own"!

Seems to me that running the Class E ur up at the limit of most commercially made ham baluns, and surely over that of the W2AU, surprised it lasted this long!

Some cores, some coax, and you could make ur own.
Maybe a worthwhile project for the cabin fever season? :D

Nice to hear you back on.

                          _-_-

What are they supposed to do again?
I never used one, what do they do?

http://palomar-engineers.com/tech-support/tech-topics/why-a-balun-by-kurt-n-sterba (http://palomar-engineers.com/tech-support/tech-topics/why-a-balun-by-kurt-n-sterba)

In my case, over 30 years ago, it helped me with TVI, BCI & Telephone RF problems.  Today it keeps RF out of the shack so I do not have problems with my SDR radios.

I tried eliminating the balun briefly and had all kinds of problem with RF in the shack.

Brett.  If you're happy not using baluns then have at it.  Baluns work for me

Al

Oh, ok.
I do not seem to have any (or much anyway) RF in the shack.
Since the antenna is right over the house, I doubt one would do much for me no matter what.

I know it sort of depends on the swr, but how much AM power is a 5kw balun good for?

Good to be back on, Bear.  I think all this could have been avoided if I had remembered that I had shortened the wire length to accommodate an inverted VEE antenna.  When I raised the ends up, the now short dipole was doing some unseemly things to the balun.

I agree I was pushing things with the W2AU balun.  I still have two other antennas up with them and am quite happy with it's performance.  This is ONLY the second one that I blew up in 30 plus years of use at high power.  First one was totally my fault.  This one?  Weellll, the antenna does have to be the correct length.

Al

Here's the math, Brett.  Also, one might have to calculate peak voltages or current given the height of the dipole at the feedpoint, length of the antenna at the frequency of use etc.  See you on 40 meters sometime.

Al

I do not think it is that simple.
I remember blowing up almost everything with a DX100 many years ago, trap dipoles, trap verticle antenna's, the B+W folded dipole smoked at 200 watts, the Alpha Delta coils melted at 200 watts carrier.
The big heathkit tuna would arc and get hot.

Voltage, current and duty cycle and SWR are a factor I guess.

Probably, just saying probably, putting ferrite around the coax at the top would keep the aRF! out of your shack,
and eliminate the need for the balun unit. I understand that QIX does this, and you doubtless have an idea as to how much powah his transmitter is capable of...

For beams on towers the solution sometimes is several turns of coax around a former, usually the diameter of a gallon jug.

               _-_-bear

Boy, Brent.  You lead an exciting RF life.  Dipole height, length when running high power become an important issue.  Mine has been pretty dull.  The station just puts out and gets out.  I should have seen the results of a 5% shorter antenna than required coming.  Live and learn.

See ya on 40

Yes, on QIX's approach.  iffen this 5 KW balun flames out, that'll be my next approach.  The MFJ 918 balun uses that approach.  It's just that the ferrite beads may be small for what I need.  Anybody have a pic of the inside of a MFJ 918?  I'd be interested in what it looks like.

Al

Here's what my 4:1 balun looks like.  5:1 is acceptable I think

Al

Well, that was a long time ago, before I just went simple.
The one antenna I never blew up was a Butternut vertical for 80 and 40 meters.
Seemed to work like a dummy load, you could dump a lot of power into it and not bother anyone.

5% does not seem like a big deal, I am sure there is that much change from one spot on the band to another.

I think one time I tried ferrites on the coax at the antenna but it did not seem to make any changes in anything at my location.






Boy, Brent.  You lead an exciting RF life.  Dipole height, length when running high power become an important issue.  Mine has been pretty dull.  The station just puts out and gets out.  I should have seen the results of a 5% shorter antenna than required coming.  Live and learn.

See ya on 40

The trick to not burning those styles up is having enough choking Z at the freq of operation.

As power goes up the need for ever more choking impedance increases.

I found this out the hard way.   Then I went to Walt W2DU directly.   

I've also used m spaced.   One at the feed point (with  maximum amount of choking impedance located there) and one base of mast / tower at the ground point,  and a particular rfi prone pc (the mouse I found out later)  required one at te back of the amplifier.

I used a string of 43 beads at the tower base and at the rear of amp.   The top of the tower was 61 or 63, two or three cores stacked and multiple turns of 393.

AMFone convinced me to go balanced,  and I haven't looked back since.  This decision,  althoughalthough,  has cost me operating time when I've relocated though.   Coass is so much easier in a rental than ladder / owl.


--Shane
KD6VXI