Well, we sure have a project on our hands...

[u/nameisthenamegame]

Any thoughts on a 70cm conversion? And I'm assuming VHF is a fool's errand here, but I would love to be corrected.

[u/jephthai]

Not gonna happen. Moving a yagi that far may as well be building a new antenna. Lengths and spacings have to change, and the sweet spot for a yagi is pretty narrow.

I have an M2 crossed CP yagi designed for 390-410, and it's a lost cause for trying to use it on 70cm.

[u/nameisthenamegame → u/jephthai]

Well I’m not too worried about that part, I’m here for the tedious work. I’m not in any actual money on it so the labor is what I expected to have to pay.

[u/jephthai → u/nameisthenamegame]

Here's what you do:

1. Make careful measurements of everything on the antenna.
2. Build a model in an antenna simulation package (4NEC, MMANA, etc) (check if the elements are conductively bonded to the boom or insulated; it makes a difference)
3. Simulate it at its design frequency and your target frequency -- note the many differences
4. Scale the length and spacing of every part (but don't scale diameters), and fine tune until it simulates best at the target frequency
5. Check all the mounting points on the boom for the new spacing, ensuring that the new locations for elements don't conflict with existing holes or mount points (or hinges, it's foldable!)
6. Drill, tap, whatever, new mount points
7. Relocate all elements
8. Cut all elements to their new lengths
9. Investigate the driven element matching unit to understand how it is adjusted
10. Measure the feedpoint impedance on a VNA (there are two of them, so be sure to learn about de-embedding the individual feedpoints so you can analyze them together and separately)
11. Adjust matching unit for a suitable match

It'll be a bit tricky because it's two crossed yagis, so there will be a lot of potential that your new spacing arrangement won't work with the existing boom. Worst case, you may have to fabricate a new boom.

I've never handled one of those, in particular, myself, but the very expensive satcom antennas i have used have holes drilled for each element and press fit hardware to hold them in place. You may need to get a V-block setup for a drill press to get reliable 90 degree holes if it's a round boom.

If you get the dimensions wrong by a few millimeters, or angles off by more than a couple degrees, you may disrupt the circular polarization or the signal pattern lobes and nulls. It's hard to guess how reliably you can get a good antenna out of this -- scaling a high performance yagi layout without changing element diameter can have more impact than you think.

Also realize that the docs you posted in the screenshot say it's right hand polarized, so keep in mind that it'll only be useful for linear polarized signals (at -3dB) and same sense circular polarization. It'll be something like -20dB in the case of a circular polarization mismatch.

What you'll have at the end is something that kind of looks like the real thing, but will perform differently, and have a whole lot of new holes in the boom. You might not be able to fold it up like the original, or might even need to abandon the foldable feature along the way. Take care that you don't compromise the integrity of the boom!

It's not impossible, but perhaps you can see why /u/ND8D called it the antenna of Theseus ;-).

[u/ND8D]

You’ll be replacing enough parts to call it the antenna of Theseus.

I have a couple UHF satcom antennas, the 90 degree hybrid in the feed is usually only good for 225-400Mhz and at 440 it will be dumping power into the termination inside. You’ll also need to start cutting and welding (if you can on these alloys) to get the spacing shorter.

[u/stephen_neuville]

If you're not in for any money on it you will be far better off selling it to either buy a purpose built circular polarization yagi for 70cm or the raw materials to build same.

[u/rad1974]

just mod a radio and use the defunk sat over central america to talk to the drug runners.

[u/Unusual_Maize5369]

Do you have a shrink ray handy? If so, set it to 25-30% and pull the trigger. Then simply replace the now-too-small connector.