This page is a collection of thoughts and pictures while rebuilding the Force 12 C-4SXL four-band Yagi, original design with linear loading. If you own an old Force 12 antenna and have contacted the new Force 12 about upgrades and parts, you know how helpful they are, and if you are a US ham having a US source for support and parts is invaluable and Force 12’s designs and quality are excellent.. They are a great company, and I’d love to own one of their new antennas. Of course I have this old one and I couldn’t resist the challenge of rebuilding it!
It seems every ham has their own ideas about how to build and rebuild antennas, and I’m no exception. I welcome any comments from others who have rebuilt their own old aluminum skyhooks, especially these old Force 12 models. All pictures on this page will launch full-resolution versions in a separate window when clicked on.
When looking into the purchase of a multi-band Yagi I was fortunate enough to first purchase and read the report written by Ward Silver, NØAX, and Steve Morris, K7LXC titled ‘HF Tribander Performance – Test Methods & Results – 2nd Edition’ available from Champion Radio Products: http://www.championradio.com/HF-TRIBANDER-PERFORMANCE-TEST-METHODS-RESULTS.2
Their meticulous testing and documentation of results confirmed what I had experienced; that trapped tribanders are not necessarily the best way to achieve multi band coverage, in fact the last triband Yagi I owned was a Moseley CL-33, and I had trap problems with it. In addition it was no better and perhaps worse on 15M than the home-brewed 15M 3-ele beam it replaced.
As a result of reading NØAX and K7LXC’s book coupled with my own experience I resolved to purchase a Force 12 C-4XL or C-4SXL 4-band multi monoband Yagi. After some looking around I was lucky enough to find a classified ad in QTH.com which read “Force 12 C-4SXL and Rohn HBX56 free if you take them down and take them away.” A phone call revealed the situation was a divorce where the non-ham spouse got the house and tower, and she just wanted to get rid of all the ham stuff. A couple of days later I was on the road with a willing friend (thanks David! We had fun in Louisville, eh?), and after a somewhat arduous disassembly headed home with the antenna, tower, and a Yaesu G800SA rotator. The C-4SXL had obviously been in the air for quite a while, the fiberglass insulators had lost all of their gelcoat and were frayed and fuzzy, and all galvanized hardware was totally rusted. On the positive side, all parts were there and the rivets were tight, showing the viability of that attachment method.
Segue to the present some, um, time later. I pulled all of the parts of the Force 12 C-4SXL out of storage and further disassembly showed the stainless steel (SS) hardware had been installed without anti-seize and much of it galled and broke when I attempted to remove it. There were numerous surprises as well: the small ends of the element were all plugged with mud-dauber wasp nests, and the boom sections were mouse hotels. I flushed all tubing out with water and tried not to blow chunks myself as um, debris came pouring out….NASTY! This picture shows the condition of the boom and other parts when starting the rebuild:
After a full disassembly and cleaning I removed the corrosion from the aluminum parts with 3M Gray Ultra-Fine Scotchbrite Pads, bottles of Loctite Aluminum Jelly: http://www.loctiteproducts.com/p/s_trmt_alum/overview/Loctite-Aluminum-Jelly.htm , wire brushes and a ton of my own elbow grease. This arduous process took a total of 5 hours, but the results were worth it:
The boom had previously been painted with some very tenacious silver paint which was mostly removed by cleaning, so the end result was clean and corrosion-free, but it looked looked spotty from the old paint residue. In order to improve the appearance and help a bit with future corrosion, I lightly sprayed just the boom and element brackets with Krylon Rust Tough Silver Metallic Enamel (p/n RTA3292). Here is the boom and element assembly after painting:
I originally thought I might switch from the original Force 12 U-bolt and PVC conduit insulator approach for element attachment, to a Stauff-type resin clamp with reinforcement plate like that sold by DX Engineering. A quick calculation showed it would cost $231 + shipping and new holes would have to be drilled in the boom attachment plates, perhaps leading to slop in positioning. Pricing out a full set of new SS U-bolts and PVC conduit came to <$80 including new wide-flange SS Nylock nuts and extra-heavy SS lockwashers, and this convinced me to follow the original route, substituting SS U-bolts for the original galvanized.
After some analysis I believe the original Force 12 method of element attachment simple as it is, would be stronger than the polymer blocks and bolts on the old-style element brackets due to how they clamp the PVC and element tubing directly against the boom brackets and close to the boom. This fully boxes the boom and makes the element and PVC tube into a structural part of the element to boom bracket. This translates the element’s rotation motion into pure shear on the boom rivets, a mode in which they are strongest, arresting bracket rotation and making the whole system tightly coupled and very rigid. It is a clever and minimalist system!
The Stauff clamps suspend the element farther away from the boom, giving the inertial moment of the elements a lever-arm farther away from the center of rotation (the boom) which can induce additional torsion on the mounting plate. Because they are not clamped in intimate contact over a large area of the plate like the original method, they offer little reinforcement to the plate stability allowing the plate to bend. You will note when Force 12 changed to the polymer block method they redesigned the boom brackets thicker, with a bottom plate to take the place of the element’s contribution to rigidity and designed them to be riveted on all 360° of the boom instead of just 180° as the original was: http://www.force12inc.com/pages/parts.html . I’m sure it is much better in general and would upgrade to it if it was worth it, but I’m also sure they could not get away with the earlier version brackets with the new mounts.
*** A note about using stainless steel hardware – many shy away from their use due to bad experiences with galling and seizing. I have used both 18-8 and 300-series stainless fasteners extensively in industrial cooling systems exposed to weather extremes as well as complex aqueous chemical mixtures for 30 years. I believe the secret to be consistent use of a high-temperature nickel-loaded anti-seize compound like the Loctite: http://www.all-spec.com/products/76732.html in conjunction with Nylock nuts. The nylon friction member creates a semi-seal at the back of the nut, holding in the anti-seize in a pocket around the threads. In those 30 years I have seldom had a stainless steel fastener system gall and fail when doing so, indeed far fewer than failure of rusted galvanized fasteners by a huge margin. One suggestion when using SS fasteners: wear nitrile gloves: the anti-seize gets everywhere and it is much easier to throw gloves in the trash at the end of the day than it is to get the stuff off your hands, plus some people are sensitive to nickel.***
There is a lot of hardware on this antenna, and I used McMaster Carr to get most of it; numerous SS U-Bolts, flanged SS locknuts and oversized SS split lockwashers and other hardware parts. They are not the cheapest source, on the other hand, there was only the one shipping charge and they showed up the next day. You gotta love McMaster Carr!
The previous owner (or one before them) had painted and taped the PVC pipe section insulators and they were in bad shape. I cut new ones from 3/4″ and 1″ grey schedule 40 electrical conduit and discovered why they were slit lengthwise: the ID is 0.085″ larger than the OD of the aluminum tubing they are supposed to clamp tightly. I ended up milling a 0.125″ slot down the length of each to allow the compression.
I began to investigate ways to replace the degraded fiberglass parts, and certainly new rods could have been purchased however the damage was superficial and did not effect the strength of the rods. I decided to sleeve the exposed sections of the 1/2″ fiberglass rod parts with 3/4″ UV-stabilized marine-grade dual-wall adhesive heatshrink tubing, and used 1″ tubing on the 3/4″ rods. Some testing showed the longitudinal shrinkage of this tubing to be about 6%, so I oversized the lengths accordingly. Here is the final result on one of the 40M element insulators:
On the subject of the 40M elements; the ‘studs’ as Force 12 calls them, clamp the loading wire to the element at midsection. As I got them, the SS 1/4-20 bolts were all seized in the aluminum studs and all attempts to heat and lubricate them failed, so the bolts were backed out by force. As they came out, so did a lot of the threads in the aluminum stud. A 1/4-20 Helicoil could not be installed, since the resulting hole was too large for a 1/4-20 Helicoil tap. As a result I just drilled and tapped them for 5/16-18 SS bolts, and added lock nuts and lock washers. You can be assured when I installed them all hardware received a liberal coating of the anti-seize, as well as Noalox between all aluminum parts!
And here is the rebuilt and heat-shrink covered linear loading spreader:
While I was planning the rebuild I talked with Force 12 about the possibility of eliminating the 40M linear loading setup and upgrading to their Tornado coil-based loading. It is a beautiful upgrade, more rugged and higher-Q, not inexpensive but well made at $189.95 ea. * 4 + shipping. A less expensive alternative would be to wind the coils myself and adjust the element lengths to achieve a match, but considering the linear loaded version works well enough and I already had it, I decided to just rebuild them. This picture above also shows the linear loading wire termination technique I decided on. The linear loading wires are 12 gauge Alumoweld, which Force 12 does not carry, so I purchased it from Hy-Gain, their part number 691067: http://www.hy-gain.com/Product.php?productid=691067
The original approach used by Force 12 when attaching the linear loading wire to the spreader was to push the wire through the hole in the fiberglass spreader and wrap it around the spreader rod, then secure it back to itself with two galvanized guy wire clamps. Given how brittle the high-tensile steel core is of Alumoweld wire, the sharp-radius bend at the hole seemed like a bad idea, and dissimilar metal corrosion with the clamps sets in very quickly, not to mention I thought this part of the loading system looked very sloppy and haphazardly designed. Of course it was probably the easiest and least expensive way to secure the wire without special tools. After thinking about how to clean up the linear loading system, I discovered perhaps the only one good thing about maintaining wire horse fencing: you end up owning tools for splicing wire. As a result I already had the solution I chose for the linear loading wire termination. Should someone else decide to do the same, the tools are not that expensive and useful for crimping large sleeves and terminals of many types. Here is a link to one such tool and gritted crimp sleeves: http://www.tractorsupply.com/tsc/search/fence%20wire%20crimping%20tool . This system is strong; I have had a 1200 lb. horse run into the fencing, and the wire did not break at the splices!
Bending the end of the wire 180° and crimping it into a #2-3 gritted crimp sleeve seemed like a neat solution to terminating the spreader end of the Alumoweld, but initial attempts showed the brittle high-tensile steel core would break when this small radius bend was attempted. While this verified my concern over the bend used with the initial design, it posed a stumbling block for my intended method of termination. This is how I dealt with this problem: I heated the last 2″ of the wire with a propane torch and watched the very end of the wire. When the aluminum there just began to dull at it’s melting point >1200°F, I removed the heat and allowed the wire to cool. This annealed it enough to allow the desired bend radius. Here is a comparison of attempted bends/crimps without (left) and with (right) annealing:
When installing the wire I placed a #8 SS flat washer on the Alumoweld under the crimp to help spread the load on the heatshrink . The final result as seen is far neater and easier to maintain than the original approach:
After installing the linear loading wires I installed the cleaned jumpers using 3/16″ stainless steel guy wire clamps and Noalox, with Nylock nuts and antiseize on the threads of course. If you order the SS guy wire clamps from McMaster Carr, it will have a little U-bolt with M5x0.8mm threads. It looks like a #10-32 thread but it is not. Buy new M5x0.8mm SS Nylock nuts to go with it! Here’s the result:
The final step in assembling the C-4SXL was mounting the baluns. I kept the EB-1s as they have >1kohms impedance from 7MHz upwards, which is acceptable. The previous owner had merely taped them to the boom, which I did not like from a stability viewpoint, and tape looks like crap. I located some SS conduit hangers and joined them, then mounted the 40M balun as shown in the upper picture below. The 20M/15M/10M feedpoint balun requires the hangers to drop the balun by at least 3/4″ to clear the 15M driver as shown in the lower picture.
This last picture shows the C-4SXL rebuild results, it came out pretty well for a 20-year old antenna. I’m looking forward to raising it and seeing how well it performs! Another Force 12 original will soon be back on the air…