Home phone:   (408) 749-8522
e-mail: doug@dougronald.com

Bringing LPDA Back

After the enormous effort I put into this antenna, not to mention copius coin spent, I virtually had to repair it. A call to USAP to inquire about ordering parts, got me to a really nice person in sales who agreed to help. I gave him a list of first, the aluminum angle, and cross braces, and got back a very reasonable quote for fabbing those. I ordered, and USAP delivered all the boom parts I had destroyed.

So here I am, about August 2020, ready to begin on LPDA number two.

Reconstructed Boom

Reconstructed Boom

The image is of the reconstructed boom with the original undamaged front sections, to the reconstructed middle and rear sections.

KLM's LP1005

KLM Airlines LP1005A

After receiving the new aluminum from USAP, I received a call from a ham in Southern Califirnia who said he just heard of my misfortune, and had an LP1005, identical to mine, in storage! I mean, what are the chances? Further, he said he would not be able to use it, and he could sell it to me for replacement parts. We agreed on a price, with his agreeing to deliver it to my ranch, and off I go. I only wish he had found out about the accident sooner.

His son delivered the most important parts, the boom (in sections), and the elements. Here it is layed out as it was delivered, ready for me to take the needed parts, and apply them to my boom.

The antenna itself was originally owned by KLM Airlines in the Netherlands, surplused by them, then purchased by the ham in So. Cal. where he shipped it from Europe to the US.

Hydraulic Winch

Backhoe Hydraulics to Winch

The 410G has an auxillary hydraulic on the boom which I have connected to the 48,000 pound pull winch. The new wire rope pull cable is now 5/8 inch, up from 1/2, just to improve my safety margin. This is the largest wire rope allowed by the antenna hardware.

Winch

48000 Pound Pull Winch

Closeup of the winch. There are two items to worry about: first is I am concerned that those bolts sunk into the concrete will pull out of the cross I-beam webs. I have the shovel of the backhoe resting on the back left side, but the right could still pull through the steel beams. Before the big lift, I am going to try and find some square steel plates to put over those bolts to at least obtain more area on the steel web. The other issue is that the whole winch is at a slight angle to the tower, and I hope the wire rope will wind on the spool correctly.

Wire Rope Interface

Winch to Pull Cable Interface

My "accident" on the first antenna disaster was caused by this interface where I had not torqued the wire rope clips to specification, and the wire rope slipped out. That is not going to happen twice. Now I have Crosley clamps on 5/8" rope, and have torqued each of these to the 130 foot pounds required. The larger cable comes up from the winch and is 7/8" in diameter, meeting my hoist cable at 5/8" diameter.

Boom Mated to Mast

Boom and Mast Truss Joined

Here is the completed repaired boom truss, now bolted to the mast plate. I have completed two weeks work thanks to some teriffic weather. Some days, thanks to California wildfires, I had to work in dense smoke, with an N95 mask for protection.

Boom Mated to Mast

Right Side of Antenna and Mast

Another view of the mated boom and mast. Getting those two mast plate bolts in was a real bear. With two people, it would have been much easier!!

Boom Mated to Mast

Pressure Test Feedline

Here is the first pressure test of the 50 Ohm coax feedline which goes up 100 feet through the center of the mast. The only leaks, and they were small, were in the rotory joint, and the 1 5/8" EIA-to-N adapter.

Boom Mated to Mast

First Element 19 Installed

Déjà vu - Been here before, with the longest, heaviest element 19 installed. All the graduated pipe sections to get to 104 feet will be for another day.

Boom Mated to Mast

Look at that heafty mount!

Well that's more like it! Someone fabricated those three U-bolt mounting plates, a far cry from USAP's flimsy single U-Bolt fastener. I guess at either Tri_Ex or maybe KLM alirlines, they too had a faliure of that single U-Bolt connection and fashioned a much better engineered mounting plate. That sucker is never going to fail in heavy winds again.

Boom Mated to Mast

Elements 16 through 19 Installed

It's Monday November 9th, 2020, the last day of incredible weather for November, and I have elements 16 through 19 installed. These are the four that have the copper loading coils, and are quite heavy to place. The nitrogen bottle is to pressure test the tapered transmission line, which passed after placing some teflon tape on one of the adapter fittings.

Not perfectly visible in this image is the completed tapered transmission line and the element feed, open wire transmission line. After using Time Domain Reflectometry (TDR) to plot the tapered transmission line's impedance vs. distance, it was clear that one section of the line was installed backwards. When placing the line in, I followed the person who disassembled the KLM antenna's markings where he had ends marked 5-to-5, 6-to-6 etc. so the antenna had to have been assembled incorrectly from the git-go. The TDR measurements were made with the VNA-2180 from Array Solutions. I altered the transmission line velocity factor to fine tune the distances, which are known. The result was 92% the speed of light, in very close agreement with the EIA 1 5/8 inch, air dielectric, hardline. The tapered transmission line Z0 begins at 50.1 Ohms and ends at 64 Ohms where it directly connects to an open-wire transmission line with a calculated Z0 of 120 Ohms. Note that is twice the impedance of the tapered transmission line, and at this point, I don't understand what is happening here. It's like there are two open wire lines in parallel?

An update to my not understanding thanks to K9WN, N6RK, and W7HPW. The dipole elements reflect an impedance which varies with physical design of the LP, and frequency, not to mention ground characteristics. Using "LPCAD" written by W8IO, using various design chriteria, I could get the feed impedance to range from 100+ Ohms to over 400 Ohms! It's that impedance which appears in parallel with the impedance of the line itself, so it all balances out in the wash.

TDR of Tapered Transmission Line

TDR of Tapered Transmission Line

Using a VNA in TDR mode, shows the impedance variation of the tapered transmission line vs. distance. Telex made the line by using sections of brass tubing of ever decreasing diameter, and brazing them together with teflon spacers every few feet. (click on the image for full size in another tab)

The plot shows a sharp up slope from 50 Ohms to 58 Ohms at the start, and there is nothing I know of to physically account for that. This is a section that has several plumbed elbows, so I need to take each one apart and find out why the sudden jump. The first small bump at 10 feet is a coulping flange interface. Even though this plot ends at 83 Ohms or so, by measuring the diameter of the brass center conductor, and the inside diameter of the 1 5/8" EIA tube, I calculate the Z0 to be more like 73 Ohms.

TDR of Tapered Transmission Line

TDR of Tapered Transmission Line

Here is a TDR plot with the correct transmission line impedance as a function of distance. The problem from the previous slide was my double elbow fix had a poor fitting center conductor which was also a smaller diameter than the 50 Ohm dimension type, and also my 1 5/8" EIA-to-N adapter had a poor connection internally. After correcting all that, I have this perfect impedance plot. (click on the image for full size in another tab)

Rear View

Back Right View

I have all elements but the last long ones, 18 and 19 completed with a mix of my original antenna's parts, and the KLM antenna's parts. None of the element-to-element connecting stainless steel bolts were with the KLM antenna, so I have used what I have from the original antenna, but I'll have to buy those that were bent beyond usability in the crash.

Business End

Antenna Business End

Front view of the LP looking at element 1 and the coaxial-to-open wire line transition.

VSWR Plot

VSWR Plot

VSWR, return loss, and Zmag of the LP just a few feet off the ground and the longest 2 elements not at their design lengths. It's actually not too bad, save for the upper frequency end. The spec is SWR less than 2:1 from 4 MHz through 30 MHz, and less than 3:1 from 3 MHz through 4 MHz. As you can see, it doesn't make that specification, but I hope it is because it is so close to the ground. Being close to the ground increases the element-to-ground capacitance, and lowers the resonance frequency, thus, the elements at the high frequency end of the LP appear longer, so the VSWR goes up. When I begin raising the antenna, the high frequency end will go skyward first, so I can do another VNA sweep at that angle and hopefully see an improvment on that high end.

(click on the plot for a larger image in a new tab)

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