10m Moxon: Ugly balun to feedpoint distance?
I'm building a 10-meter Moxon (18ga. wire, fiberglass spreaders, pvc mast mount, thin dacron rope to keep things taught---and no metal parts other than the elements and coax) and want to also use an ugly balun (18 feet of LMR-400) on a 3.5" PVC pipe section.
If I mount the balun right at the feedpoint the weight will be a problem; I'll have to construct an arm so as to suspend the rather heavy balun (3" grey PVC is heavy stuff!) from it so the feedpoint isn't sagging. The balun would also be in the same plane as the elements (not sure if this matters or not).
If I mount the balun vertically, say cable-tied to the PVC mast right beneath the center mounting plate (plastic cutting board) this will result in a much easier to construct antenna.
Will this 24-30 inch (approximate) distance from the feedpoint to the balun be a problem if mounted this way?
If anyone has some insight into my minor dilemna I'm all ears.
I'm really trying to keep the total weight down so I can get it way up in the air by myself (using those portable "section" masts with the top 6 feet being all PVC)---so I'm leaning towards mounting the balun on the mast.
Whoa there young fellow
I hope you have great success: yes many have had the same problems you face. All antennas are a compromise. Even those at N5UA or W8JI where multiple 200+foot towers stand are subject to what's possible with what's available. Perhaps these are much less of a compromise than you or I have to make but I doubt anyone would say they never had to decide between two opposing choices.
Originally Posted by NH7RO
In your case I see where you are using 18' of LMR 400 for a decoupling coil and I ask, why? I know it has less loss than most coax but how much less? Perhaps a lighter weight slightly more lossy coax might help you with a good compromise. If you are going to use maximum power (1500 watts) you can't make this compromise because power handling capability trumps weight in that circumstance.
Still the charts I've seen show that a light coax like RG-58 has 3.1 db loss per hundred feet at 50mhz. You are only going to use 18 feet before you can switch back to a lower loss cable. So .18 X 3.1 = .558db loss at 50mhz. Since this is a 10 meter antenna the loss will be lower than that or less than 1/2 db.
You now have to ask yourself if this makes a good choice in your circumstance? Is 1/2 db a big deal? Will you need a cable capable of full power? Does the weight advantage trump these other disadvantages? If those mean this is a bad idea for you, there are other possibilities, perhaps you would prefer an iron core balun, some of them are lighter than that much LMR 400 some are not, they all cost money.
You mention lowering the coil to the mask, that's not bad either just try not to allow too much lead and keep it at right angles to the elements.
If there is a will, there's a way. Follow good contruction practices, try to make compromises that don't cause too much damage and it's likely you'll find a good preforming antenna.
Choke balun choices
Thanks for your insight, K5BO. You make some really good points about lighter coax and power handling and so on. I neglected to mention that I'll only be using this with 100 watts for the foreseeable future; I chose LMR-400 as it's among the best of the smaller diameter coaxes out there.
I was planning to have one continuous run---inclusive of the 18-20 foot balun winding---from the shack to the feedpoint which could end up at around 110 to 125 feet (as best as I can determine). I just received a 130' long run of LMR400 that I ordered expressly for this purpose.
I'm aiming for a height above ground of 28-35 feet and the mast is roughly 55-65 feet from my shack. No rotator, just Armstrong Method for the time being.
Should I elect to go with a thinner, lighter and slightly more lossy bit of coax for the balun I will also have to absorb the slightly extra loss by using more connectors in the path, too. Perhaps I'm getting too pre-occupied with these trivial points but until recently I've been a 6 meters/VHF and up guy---so I know that every little fraction of a db here and there can add up quickly. I live for DX, too, so what I can hear is of utmost importance.
The only RG-58 type of coax I have on hand is some weathered 12 or 13-year old RG8X that's seen better days. If I decide to try it out I might be better off getting a brand-new hunk of LMR240 or equivalent as I'll also have to order it online---nothing like that available locally (rural Hawaii) for me nowadays.Our only Radio Shack on this side of the island sadly doesn't stock RG58 anymore.
One further question---would the thinner coax require a different length for a choke balun? Probably not, is my guess. I tend to see 18-21 feet listed for an ugly balun just about every site I've visited.
The weight factor is important to me but not the absolute end-all*. I understand that every antenna and situation almost invariably requires a compromise of some sort.
The PVC form I'll be using for the balun are three 3" PVC couplings fitted together(11" long). Their outside diameter is 4" and provides a circumference of 13". Weighs in at a hefty 2.5 pounds without the coax. I've got a few PVC fittings on hand that I can construct a short "balun boom" that will extend from the mast out to just above the feedpoint should I decide to suspend the monster there instead of close to the mast. That would add another pound or so but provide a decent solution to mounting it close/at the feedpoint which is probably the best way to go, performance-wise.
At any rate, I'll be careful to make sure the balun is below and at right angles to the elements.
Will keep you posted on what I end up doing. Since I have everything I need to go ahead with my original plan that's what I'll probably try out first (next week or two when I have some time off).
If it doesn't work out so well then I'll try something diferent like a thin-coax balun or ferrite beads...
(*Since I am prone to perfectionism (or sheer lunacy) I plan to drill a bunch of holes in the PVC to reduce the weight a tad. That's what I did to the 11x14.5" cutting board already and shaved off half a pound in the process.)
Do you realise that a balun with those dimensions will have almost no choking impedance on 10m - it's far too large. Take a look at the chart here:
Something like 5 turns would be far more appropriate for 10m.
A ferrite-cored solution would be much more effective and possibly lighter still.
Put the choke at the feedpoint. Make it from lightweight coax if you are only running 100W. Skip the PVC choke holder. Wind it as an air core solenoid 4-5 turns about 4in diameter and use some zip ties to hold each pair of two adjacent turns together. This is only 4 or 5 feet of coax.
I don't know why you want to use PVC at the top of your mast. I would not. I'd use 4ft long aluminum camo net support mast sections all the way to the top. There is no reason I can see why you would not use conducting mast to support the moxon. Its lighter, stronger and interchangeable with the other mast parts.
That's terrific news for me, Steve---I was wondering about the right length or number of turns until now---I'll go with 5 turns on a 4" form which will also be much lighter in weight and simpler to mount right below the feedpoint. Thanks!
Originally Posted by G3TXQ
Have you had a look at the Moxon users web page ? If you haven't, do a Google and read what the others around the world have been doing, it will be very helpful for you
Yes, I have scoured the Moxon Antenna Project website* as well as others---great resources there and lots of photos of various installations.
(*which includes the mention of no absolute need for a choke balun with low power; but usually best for a clean pattern)
Hopefully, my modest Moxon will eventually join those excellent examples.
I agree adding connections will cause what's known as insertion loss. Your aversion to this shows experience in VHF, UHF or micro wave systems.
Originally Posted by NH7RO
Still you should understand that insertion loss is somewhat frequency dependent. A study done by VK3JEG compares the insertion of two N-type connectors and a barrel joint with two PL-259 connectors and their respective female joint. Not surprisingly the N connectors well out preformed the PL-259 at frequencies above 50mhz. However at 50mhz and below insertion loss was pretty much zero for both.
So though I see how you would feel and know that your concern is well based on the short wavelengths, on HF inserting a coax as a 1/4 wave matching line or choke coil is not near the problem as at higher frequencies. Even ancient PL-259 connectors should suffice.
Someone should not take this as me saying splice a bunch of short peices of coax to make a longer one. But a single splice won't make a noticable difference.
Last edited by K5BO; 08-28-2010 at 03:03 AM.