The AM Forum

The stars have aligned and my long awaited Tennadyne LPDA T8 should be resting comfortably atop the roof today!

It has been a long time coming, what with reading all I could find on the subject, building, checking, building again, waterproofing, bug & bird proofing, aligning strong young backs, priority family obligations and the weather (it is currently 26 degrees here). I have no tower (live next to a general aviation airport) so the antenna will be mounted on a piece of black galvanized pipe protruding through the roof. It should, hopefully, end up about ten feet or so above the peak of the roof.

I'm hoping the neighbor kid, a budding photographer, will be around to get some installation pictures. I'll post a few later on assuming it (and I) are still in one piece.

get 'er up there before the real antenna WX starts....this is just the beginning!!!!!
Fred

You got it, Fred. My blood is definitely getting thinner as I age!

Putting antennas and towers up is a look of work with lots of planning.  I know you will reap the rewards once its up in the air. 

I am assembling the pieces to put my new to me used Heights crank up tilt over tower up.  Unfortunately with the other projects around here, it will have to wait until the spring. 

Joe, GMS   

Nice antenna! You will have fun on the high bands.

One day delay in game penalty.

Winds went from calm to gusts over 30 Kts in no time flat! Hopefully tomorrow early morning will be better!

Pictures of installation. I was the guy on the ground on the other side of the camera :D
Second set of pics to follow

Congratulations John.  The T-8  is a very good antenna.   A friend had one up for a number of years,  and worked all over the USA and made many DX contacts,  mostly barefoot.

Hope that you have  coax attached to it.  The upper bands have been quite good lately.  73  Have Fun,  Vic

Hopefully this, the final photo, will take

Notice just below the attachment box: Maximum attachment size allowed: 5000 KB, per post: 3

The one picture you attached was 4750 KB.
Lower the resolution of your pictures. They'll load faster and any one on dial-up or slow DSL won't P&M either that they take forever to load or won't load at all.

Here's one of my antenna pictures as a sample; it's only 36 KB.

(http://amfone.net/Amforum/gallery/25_13_02_07_1_41_35.JPG)

I commend you on your wish for good resolution.  Given the file size limitation for each post as established by the forum, there is a way to keep your resolution by compressing your JPGs without appreciable loss of detail.  Check the uploaded file.  The reduction in file size is by a factor of 10.  This is possible by a utility called "Irfanview" - it is freeware meaning there is no cost for personal home use. 

It was becoming a favorite of some at my facility at Raytheon as a means of reducing file size - the IT people saw the value in this and made a financial arrangement with the producers of the program for proper licensing of the program.

All you need to do is load a given file and save it at 50% compression.  I cannot see any degradition in your file after this file reduction.  Check it out - the picture size is the same as yours - I have uploaded it for your viewing

Al

Actually, to cut a fine point, a setting of 34 % yields a 10 times reduction in your file size.  Again, no appreciable reduction in detail

Al

Fantastic job on getting the antenna off the ground and on your pipe mast.  Looks good and now give us some feedback on how its performing!

Have fun....

Joe, GMS

Nice ant OM!

All the LP antennas I ever installed always had the coax running inside the lower boom section.  Your coax is attached to the outside of the lower section,  this can cause some unbalancing.  Is some cases the coax was attached near the front and then dropped on an angle to the mast.  Maybe non of this is possible with your antenna.

Wondering what the instructions recommend about this issue, if it is even mentioned?

Fred

Fred,
I just got on the manufactures web site and you can see how they recommend routing the coax. 
Joe, GMS

A. Thanks to all for the advice about loading attachments. I appreciate it and will make sure they shrink a bunch from now on!

B. There is a LOT of controversy about the proper care and feeding for an LPDA, especially that from Tennadyne. After a whole bunch of reading I decided to go with the sage wisdom of K6ELQ. The issue boils down to this: The coax connects to the front (leading edge) of the antenna. The manufacturer places a standard 'Collins' balun at this point. The Coax then runs along the bottom boom to the mast. This causes the signal to again be coupled to the boom! In the K6ELQ solution, a second balun is placed on the mast, below the boom, so as to once again decouple. The coax never passes through the lower boom as the design precludes it doing so; the elements feed through the boom, taking up all the available space.
I am definitely not an engineer but the logic worked for me, so I went that way. There are numerous web posting from guys who used the single balun at the feedpoint and had horrendous SWR issues. At any rate - it worked great for me (<1:2) except for a small SWR bump on 17M where it pops up to 1.5:1 or so.

I'm going to try to attach the article by K6ELQ (at the risk of doing something wrong here too. The properties of the file say it is 2.33MB, so hopefully it will work OK.

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!

Since both booms are active parts of the antenna, I would think running the coax along the bottom boom is not ideal. It might affect performance, resonance points, etc. I would have just attached a PL-259-type right angle connector to the end balun and then just drooped the coax over to the mast. But, maybe on HF it's less of a problem coupling the coax to the lower boom.

You are spot on, Pete. That has been proven to improve performance with a single balun. Coax along the lower boom definitely causes coupling issues. The reported issue with drooping coax is the propensity for the dangling coax to get windblown and tangled and/or cause varied performance as it gets blown about.

Take a length of rope and tie it to the lower boom near the balun. Run the other end of rope at roughly a 45 degree angle and tie it to the mast. Make the rope taut. From the balun, loop the coax down to the rope roughly 1 foot below the antenna. Use plastic ties to hold the coax to the rope. May not be pretty, but it should work.

With the apparent problems with running the coax on the hot boom, why does the manufacture recommend that method?  You would think by now after making that beam for some time, they would have recommended something different.  I am sure they have received lots of phone calls about that scheme. 

The last time I used an LP was in the Army during the early 70's at Ft. Gorgon, GA.  I all remember is a tower section was used as a boom and it went down to 40M and worked excellent.  I have no recollection on how the coax was run on that beast but probably inside the tower / boom section. 

Joe, GMS

The original owners of Tennadyne said they never tried toroidal baluns and therefore could not recommend them. The new owner is far more customer oriented, builds a much more mechanically reliable antenna and..... wait for it ..... has never, so far as I know, spoke up about it one way or the other.

Like most facets of our hobby, there is likely no single 'correct' answer. My extraordinarily brief experience indicates the dual balun solution is working FB.

With a lot of help from guys here who know what the are doing I got the following photo down from 5MB to 500KB. Nota s small as some others, but a sizeable reduction

Part of the theory with running the coax inside the LP boom is that there is no need for a balun as the boom decouples currents from running on the coax shield.  Getting the coax inside the boom is certainly not easily done.  The coax needs to be in the boom before the elements are attached.  BUT, the antenna needs to be designed in a way to allow this.

The coax would exit to rear of the boom and then dropped on an angle to the mast.  This causes the coax to be free dangling to the mast, another problem.  I think this method allows the antenna the best performance.

I'm thinking that a piece of tubing attached the the lower boom could have the coax run through it.  Never seen this done but I see no reason why it wouldn't work the same as running the coax in the boom.

Fred

Fred,

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

The balun provides a nice interface between the antenna and coax and also prevents RF currents from being propagated down the shield of the coax. However, it seems to me that if the coax coming out of the balun  is then strapped to the lower "RF hot" boom for a good 8 or 9 feet before it gets to the mast, the potential for RF propagating back onto the shield of the coax is high, which is something you don't want. If my rope idea didn't appeal to you, you can hang a length of PVC tubing (secure it with maybe, 2 or 3, three inch screws) below the lower boom and run your coax through that back to the mast.

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

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

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

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.

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

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

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.   :o
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

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.

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   

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.