LPDA Going Up Today!

[KA2DZT]

The coax should not be near the boom.  At HF freqs, having the coax only a few inches from the boom isn't much better.  Meaning, running it along the boom on some sort of plastic hangers doesn't improve anything.  Running through PVC is the same as tie wrapping it to the boom.  I think, if it is run along the lower boom inside metal tubing, the tubing being attached to the lower boom, would work.  This probably is the closest thing to running it inside the lower boom.  The coax should run all the way to the back end of the boom and then dropped to the mast.  Probably, the coax could run inside the added length of tubing to just the mast where it would exit the tubing and drop at a right angle down the mast.  When the coax is inside the boom, it usually exits at the rear end of the boom.

Having the coax inside the boom decouples currents from running on the outside of the coax, therefore no real need for a balun.   Using a balun along with  the coax being inside the boom may be the overall best way to feed the LP antenna.

The feed point impedance of the LP should be around 50 ohms.  This can be adjusted by changing the spacing between the two booms and adjusting the length of the shorting stub that's at the rear of the booms.  Most all LPs will have that shorting stub at the rear of the booms.  Although, it's not always at the very end or beyond the end.  Sometimes it's just a shorting bar across the two booms at or somewhere near the end.

Fred

[KA2DZT]

Fred,

Would that apply to a dual hot-boom configuration as is the case with the Tennadyne?

Yes, most all LP antennas are constructed using two hot booms like the Tennadyne.  There are other ways to construct LP antennas but the dual hot booms method is the overall easiest to construct.

BTW,  I think your antenna looks great and don't worry about all this coax stuff.  As long as the antenna is working OK with the two baluns, that's all that counts.  Few hams have 100% perfectly installed antennas.

Fred

[flintstone mop]

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BTW - I know this is straying rather far afield from A pure AM discussion. Please let me know if it is time to cease and desist!

Nope just about anything goes here. The AM mode needs to involve electronics, antennas, RF, radios, tubes and solid state. Pretty much anything Ham related.
There was some booo-hooing from members here when folks wanted to use a "plastic radio' driving a linear amplifier, or the latest Software defined radio craze. It doesn't have to be a plate modulated DX 100 or a Gates BC-1T.
Your project looks great! Now you're ready for some radio.
Fred

[WA2OLZ]

Lots of great ideas from the members here; all much appreciated. For now, I will leave the antenna as is on the roof with the two baluns installed.

Pete, the balun on the mast is there for that very reason.

So far, so good. My only worry-wart concern now is that I forgot to apply Coax-Seal to the balun connection at the feedpoint. I hope that doesn't bite me before next spring. It takes a small army (see photo in post #21) to get up there, lower the antenna and make the coax connector accessible.

[flintstone mop]

It would be nice to see a picture of the feedpoint. It seems to be an openwire connection, like a TV antenna used to be with the 300 ohm stuff? As long as there is some space between the terminals it should not bite you. The space would prevent ice forming between the two connections.
Fred

[WA2OLZ]

Hi Fred,

Sorry to say I did not take any photos of the feed as I was building, although I should have. However, look at the .PDF attachment in post #16 above. There are some excellent pictures there that show the scheme. The elements have a fixed  air gap between them and the mounting studs at the element ends are about two inches apart. The whole works where the balun is mounted is well protected with a couple of layers of high quality silicone.

-Jack

[flintstone mop]

VERY nice pdf. Very nice pictures and good detail.
I have never seen a design like that before. A balun does the honors right at the boom.
OK there is an SO-239 on the balun for your radio.
So you say you did not do the shrink tubing weather seal on the coax.   
You should take care of that when the WX breaks and you have lottsa energy in early Summer 2014. PL 259's are not water proof. Type-N connectors are.
Hopefully the braid will not start to take on water. That trickles down the entire coax and makes a mess inside.
I quit dreaming of the ultimate antenna and tower after we moved to Pa. in 2004.
Fred

[WA2OLZ]

You are 100% on point, Fred. My screw-up. It will be weatherproofed not later than Springtime. Worst case is a replacement length of LMR-400-UF from feedp oint to the mast. Mea cupla.

[W1ITT]

For what it's worth, I have installed a number of 4-30 mhz rotary log periodics in various parts of the world.  They were paid for with your tax dollars.  The boom on these monsters is about 100 feet long, and about 3' triangular lattice section.  The elements are sawtooth wires tensioned to a catenary that goes from front to back.
The feedline is open wire "line" composed of juxtaposed angle stock, and that feeder is run out through the center of the boom to the high frequency end, and then to a wire balanced line going back towards the low frequency end, from which the sawtooth elements are fed.  The important thing to notice is that, at all times, the feed is isolated from the elements until they are fed.  For coax fed applications there is a big balun at the tower top and a rotary joint.  For all-balanced applications the feeder twists plus and minus 180 degrees in the vertical run.  The ones I'm familair with run about 200 ohms in the balanced section. The picture of the coax running back along the bottom boom scares me a bit.  But if Tennadyne tells you to do it in a particular fashion, do it.  As I tell the customers, RTFM.  I'd consider a ferrite common mode choke in the rotor loop.
Once, after putting up a couple of these in Puerto Rico, I put a 50 watt ssb rig on 75 meters.  Turning the beam up to New England, it strapped.  LP's "want" to work, and quick band changing is definitely a bonus.  I'm still waiting for the boss to have a "scratch and dent" sale so I can have one in the back yard.

[W3GMS]

For what it's worth, I have installed a number of 4-30 mhz rotary log periodics in various parts of the world.  They were paid for with your tax dollars.  The boom on these monsters is about 100 feet long, and about 3' triangular lattice section.  The elements are sawtooth wires tensioned to a catenary that goes from front to back.
The feedline is open wire "line" composed of juxtaposed angle stock, and that feeder is run out through the center of the boom to the high frequency end, and then to a wire balanced line going back towards the low frequency end, from which the sawtooth elements are fed.  The important thing to notice is that, at all times, the feed is isolated from the elements until they are fed.  For coax fed applications there is a big balun at the tower top and a rotary joint.  For all-balanced applications the feeder twists plus and minus 180 degrees in the vertical run.  The ones I'm familair with run about 200 ohms in the balanced section. The picture of the coax running back along the bottom boom scares me a bit.  But if Tennadyne tells you to do it in a particular fashion, do it.  As I tell the customers, RTFM.  I'd consider a ferrite common mode choke in the rotor loop.
Once, after putting up a couple of these in Puerto Rico, I put a 50 watt ssb rig on 75 meters.  Turning the beam up to New England, it strapped.  LP's "want" to work, and quick band changing is definitely a bonus.  I'm still waiting for the boss to have a "scratch and dent" sale so I can have one in the back yard.

You description sounds like what I used down at FT Gordon, GA in the early 70's.  I may have a picture of it somewhere around here. 

Joe, W3GMS   

[Steve - K4HX]

I've used on the order of 20 different LPAs over the years, including log spirals printed on a flat sheet of mylar. Of the three dimensional types, these were almost all designed for EMI measurements or milcoms. The EMI units came from manufacturers like Watkins-Johnson, AH Systems, Antenna Corporation Of America, Rohde & Schwarz, Amplifier Research, etc. These came with calibration data and antenna patterns. Feedline radiation would have obviously been a problem. As best I can remember, all but one or two of these used a dual boom design that also served as a feedline. There was no coax involved. The ones that used coax, the coax was run inside the boom. The coax connector for the antenna was always at the back side of the antenna.