1:1coax balun Help Please

[Marconi]

Help Please. I have a Maco 5/8 GroundPlane. Using 65 FEET OF LMR400 Coax. I recently made a 1:1 Choke Balun out of 6 inch Diameter Pvc piping. I used 25 feet of extra LMR400 And only wound 21 feet 10 turns of it on this pvc piping. single Layered only Leaving 2 feet of coax coming out on either end. Tested this Balun before putting her up with my 1500 watt MFJ dummyload and got a Flat Match on my Astatic 600 Meters.I plugged one end to my Maco and the other End I used a coupler to join them together. The wound coax is not in any way touching my mast because of the 3 inch legs on both ends of the pvc that hold it away. I noticed My match is higher and I'm now causing Ripples on my TV set which is satellite Controlled. Match with my box on is now close to about 2 where before it was Flat. I took her off and thing went back to normal. Is there a reason Why this Happened. Please any Advise on this Matter would be greatly appreciated. Cant Upload a pic.Too big. I can Email pic if needed. Thanks.

Thanks Thundir, excellent thread and lots of good conversation to make it all work. Lots of good ideas and thoughts about our interest. I for one hope we addressed your issue.

 

[Marconi]

My response to BM's question

i have 50 feet of 8x tramflex so it would have a lot more room to shift around in a 3/4 tube so it seems that would be asking for trouble . but if i used some of that courrgrated plastic wire loom (like is used on car wiring systems) on the coax inside the tube that might keep it fairly consistantly centered.........but what are the odds that spacing would be remotely acceptable for 27MHz. ? from what i understand a roll of coax can be off 10 or more ohms in some places so it seems proper spacing is critical for correct impedance . is there a formula to figgure that out ? trial and error ? antenna analizer ? luck ?

lmr-400 is only .405 inches outside diameter so it still has .345 of play to shift around inside the 3/4 tubing .

marconi , you mentioned "I think it could simply work in the case of the Starduster just to shield the first 1/4 wave of the feed line from the inductive RF (not CMC) of the ground radials......."
since the tube is clamped to the outside/ground of the 259 doesnt it itself become a ground element that could radiate inductive RF in very close proximity to the coax essentally encapsulating it inside it ?

since ill be upgrading to a 5/8 that really shouldnt need this im %99 sure i wont be attempting it , but id still like to try to understand it a lil more and ill thank you guys again for the great info y'all have shared here and putting up with me . seems every answer i get leads to more questions . but i think thats a good thing .... as long as i dont get on peoples nerves looking for answers ! hahahahaha .

Booty, regarding your question to me. Since the SD'r mast is connected at the hub and grounded as you correctly note, it likely responds nothing like the balun being discussed here. The construction is wrong for that purpose. So, my comments were an attempt to explain a possibility for what was happening, because the coax exists inside the support mast and that is similar to the sleeve balun idea. This idea of mine is mostly based on my reading of the references I provided in my previous post, the Patent and the ARRL article. If it does not fit the true idea for the bazooka balun as described in the article, and yet we did see a positive response when we grounded the coax shield at the bottom of a modified 9' long tube attached to the side of our support mast. TVI went away. It could have been due simply to adding the extra line also, and then it was all a coincidence. But we rationalized that maybe the tube was shielding the coax from RF off of the radials a bit, at least for the 1st 1/4 wave down from the feed point and to a point near the bottom of the slanted down radials. Maybe that made the positive effects, so we thought.

Half of your closing remarks concerning my comments are correct, but I’m sure that RF follows skin-effect rules. So, there should be no inductive RF from the down radials reaching this portion of the coax itself. CMC would be another question however. The outside of the SD’r tube would arguably be a part of the radiating element to the extent that is possible, but the coax inside would seem to be to be shielded. Back in the 80's, BM, I did not even know that CMC currents existed, and the idea is still a bit of an enigma even today in my mind. This was my point. Thanks for clearing that up.

 

[bob85]

booty,
yes the sleeve connected at the pl259 will radiate,
shorting it at the opposite end providing the sleeve to coax diameter ratio is large enough should give you a none radiating sleeve with a decent choking impedance but only at the frequency you design the sleeve balun to be used at,
move away from that design frequency and choking impedance drops,

there are calculations that will give you a fair idea of the characteristic impedance of the transmissionline formed by the sleeve and coaxial outer braid,
you need to know the dielectric constant of the space between the coax and sleeve ( including the jacket around the braid ) the inside diameter of the sleeve and the outside diameter of the coaxial braid,
this is not the choking impedance, it is the characteristic impedance of the transmissionline that has a direct bearing on the choking impedance you could expect with a sleeve of the correct length,

heres something to read that may help you, take it for what its worth as i have not cross referanced it to verify accuracy of information, theres so much misinformation around the www it aint funny,

Cable impedance

capacitane/inductance calculations

 

[BOOTY MONSTER]

thanks again guys . im gonna let this marinate in my brain and get a better grasp on whats been shared so far . then........ I'LL BE BACK !!! hehehehe

 

[LightFoot]

Thank you Marconi I noticed there is lots of response. Thats Great. Lots of information. I mean Really Good Stuff there. Also thank you to everyone that Participated in answering my Question.