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Tom,
You are definitely on the right track.
I would suggest that you read the Balun Designs reference, above, w.r.t. counterpoises, radials and grounding.
Two other good papers to read are ones written by W8JI at
https://www.w8ji.com/counterpoise_systems.htm
and
https://www.w8ji.com/fcp_folded_counterpoise_system.htm
If you would consider using an inverted-L configuration, you could lengthen the wire and it will still fit your property. Then, you can ground the UNUN to ground rod, or install radials, or install a counterpoise.
You may also want to install a common mode choke or two. They should be installed where the common mode currents are expected to be maximum on the coaxial line. Placing them at arbitrary positions on the line is of little benefit.
All that having been said, your antenna can be modeled in EZNEC which is now free to download and use. If you have never used EZNEC, I can try to help if you would like.
Have fun.
Marty, K1FQL
Hello Tom,
You have correctly chosen a non-resonant length to use with your 9:1 UNUN. Strictly speaking, #12 AWG exceeds the National Electrical Code for wire antennas shorter than 150 ft! What it says in 810.52 for transmitting antennas for wires shorter than 150 ft is that you may use #14 AWG. The wire may be either hard-drawn copper, copper-clad steel, bronze or other high-strength material.
The length that you specified should work without much correction in length. There would not be a large adjustment for velocity factor due to insulation. I would advise you to begin with a slightly longer antenna length. Try pruning it down a little bit to see if you can improve the match at your desired center frequency. Even so, you will probably find that there is still a significant mismatch that will require the auto-tuner in your rig or an external antenna tuner near your rig.
I am assuming that you chose the 71′-10.25″ length based upon available space. There is a nice table of non-resonant lengths at the Balun Design website at https://www.balundesigns.com/content/Wire%20Lengths%20for%204%20and%209-1%20ununs.pdf
Good luck with your antenna.
Marty Blustine, K1FQL
Hello Anthony,
Our posts must have crossed. That 46′ mast was one of the ones in my last post. Those masts are meant for temporary installation. It is not enough to drill holes in it for drainage. Any holes drilled in the tubing will weaken it. The water has to be prevented from entering the sections where the clamps are, and that would require you to tape them up. The tape won’t stay on forever. On my antenna range in Florida the tape only stayed on for 3 weeks.
I made my flag pole mast by “splicing” sections of fiberglass tubing together with 2′-long fiberglass sections of tubing the next diameter up so that there was a sliding fit. I applied RTV on the upper part of the sleeve, only, so that I could take the sections apart. The RTV on the upper part of the sleeve prevented water from running into the joint from the top. I found that capillary action from the bottom of each sleeve was not an issue. Since the sections of tubing are butted together, the weight of the mast is borne downward and the structure is in compression. If you were to use some side mounting brackets on the tubing, that might take care of any lower guying that might be necessary, but an upper guy might be required.
If in doubt, you could always hire a PE to do an engineering calculation for you because you are going to attach the structure to your house. They might also analyze any shear and recommend what adhesive should be used. You never can tell – the PE might tell you not to do it at all! A 40′ mast has to be really strong. Please don’t underestimate the wind loading of a piece of large diameter tubing. It is far greater than you may think.
Maybe, when your spouse hears what you have to go through to attach something to the house, they may have a change of heart about a tower for which the structural analysis will have already been performed. Maybe, you could put up a free-standing tower on your lot as a compromise? If you think a tower is expensive, you might consider the cost of property damage if something that you built were to come down.
There are plenty of good, used tower sections for sale out there, so the cost of a tower should not be an issue. You might even find someone who will give you a self-supporting 40′ tower if you help take it down and haul it away! Try the next flea market or hamfest. Best of luck. K1FQL
Did a bit of hunting.
Here are two options. They are a bit pricey.
https://www.dxengineering.com/parts/dxe-tfk46-hd
https://www.dxengineering.com/parts/dxe-tfk46Hi Anthony,
You could use a metal telescoping mast like the one that Rohn makes, or you could make a heavy duty fiberglass mast of your own.
I made one from tubing purchased from DX Engineering. I show pictures of it on my QRZ site, and I have attached a PDF file below.
You can also buy telescoping metal and fiberglass flag poles from numerous suppliers. There are still plenty of options in spite of supply chain issues.
This is my first response to a post on the club site, so I don’t know if it is instant or has to be moderated.
Good luck. Marty K1FQL
Hi Anthony,
Light duty fiberglass masts are not a good idea for semi-permanent installations. I some experience using them for antenna testing over a period of weeks through rain and high winds in FL. If you do decide to use them, I’d suggest that you tape all of the joints with electrical tape to keep the water out of the sections. It is really bad news once water gets inside these masts.
The other downside is loading. They are really meant to support vertical wires for such things as end fed verticals and end fed half wave antennas for which they are plenty strong enough for short periods of time if you do not use the fragile top sections.
When I used three vertical fiberglass masts in a row for antenna tests, guyed all of them at two levels and used back-guys on the outer two in order to offset the horizontal wire loading. Please see my QRZ page under for photos (link below). Inverted Vee’s are usually okay for a short period of time.
Have you looked for masts from Spider Beam and Max Gain Systems?
I would have suggested a metal mast, but you state that your antenna has to be OCF. That would not be such a good idea to put a metal mast near a voltage point on the dipole. A metal mast might be okay for a center fed dipole in which case the support would be near the current point on the antenna. Rohn makes a metal, telescoping 34 footer and you could raise it an additional 6 feet off the ground. Grounding the mast would be a very good idea, particularly if you don’t want to void your fire insurance. You could also buy metal or fiberglass mast sections and make your own which is what I did for my flag pole antenna.
I made a flag pole antenna sleeve from fiberglass sections purchased from DX Engineering. There was a 6BTV vertical inside the sleeve. DX Engineering sells very strong fiberglass tubing. You can see what I built on my QRZ site. I had my flagpole up for over two years without any problems. I could raise and lower it during hurricanes. Sections of the antenna were assembled with short sleeves and RTV adhesive to keep water out of the sections while still allowing for disassembly at some of the joints. Raising and lowering a heavy 40′ mast will be a bit of a challenge, but that is to be expected. If you paint the mast with a good marine enamel like the kind they use of fiberglass hulls, the mast will last a lot longer.
How about a telescoping flagpole used as a mast? Could be metal or fiberglass. There are lots of suppliers for those on the Internet. There are still a lot of options in spite of supply shortages.
Let me know how you do.
Regards, Marty, K1FQL