Isolating 2m/70cm across the base insulator of an HF antenna

[Batman]

looking to use a device similar to this FM Isolator to isolate a flag pole mounted omni dual band (2m/70cm) ham antenna from the flag pole which is the radiator for an HF (160-6) antenna system over a buried radial system.

anyone ever try this?

[kf4sqb]

I don't know if I would want to try that. By isolating the V/U antenna, you've just made it into a lightning magnet. With it isolated, it will no longer be DC grounded, and any near-by lightning strike will very possibly induce a large enough voltage into it to cause serious damage to your equipment in the shack. You also have to look at the fact that the tip of the flagpole/antenna is the point of max RF voltage, and would require a pretty extreme insulator to prevent arcing, especially down in the 160m band, especially if you're running anything over 100 watts or so. The V/U antenna mounted there will also, even if insulated, at least slightly, de-tune the HF vertical.

[Batman]

by that logic am/fm broadcast facilities would be a lightning target and fried equipment would result. because this is what they do when running a multibay FM array on a hot am medium wave radiator.

the tip of the am radiator would also be the point of max rf voltage.

i know am stations use spark gaps on the am radiator on the fm probably some kind of gas discharge arrestor.

but this is done in commercial broadcasting more often then not.

i would say it can be done with other frequencies as well.

but i guess it's something hams have not had any reason to mess with.

i'm definitely going the flagpole route on HF. maybe i can hide the VHF/UHF in a support mast for some bird feeders and bird houses.

[Al]

I've used isocouplers to couple an AM 88 degree(quarterwave) tower with a side mounted FM multibay antenna. The LF feed arrangement doesn't change. The FM isocoupler is just a single turn high current transformer with air dielectric(low interwinding capacitance) and a nitrogen bybass to assure that the upper(tower-mounted) transmission line section receives the same pressurization as the isocoupler-to-transmitter line section.

[Batman]

i'm not even getting any where near the kind of power territory requiring pressurized coax.

top out at 100 watts pep.

[kf4sqb]

Robert, I'm not saying that what you want to do can't be done, just making sure you're aware of at least some of the possible problems.

I understand that commercial broadcast companies do, more or less, the same thing you want to do, but their systems were also designed by RF engineers, and built with equipment specifically designed for the purpose. I'm going to guess that that equipment is also very expensive. Also, with a commercial broadcast tower, the FM array is, most likely, going to be "side-mounted" a short ways down from the top, not sticking straight up from the top. I would think that this would afford it at least some protection from lightning, as the tower would be more likely to take a hit than the FM array. I'm sure that there have been quite a few Hams that have tried similar things before, especially those in antenna-restricted areas, like I'm guessing you are. If you haven't already tried it, Google may be your friend in this matter, although I have no idea exactly how to word a search for this. Either way, feel free to experiment, that's half the fun of Ham radio! Another option, however, may be as simple as installing another flagpole to either be a V/U antenna, or be a support for one. Put the American flag on one, and the state flag on the other?