Tuning a 4m yagi to 3m?

[BlackBeard]

Hello,

recently, I was asking myself if I could tune this yagi: https://www.sirioantenne.it/en/products ... -3-sy-78-3 (the Sirio SY 78-3) to 91 MHz. It was designed for 78-88 MHz which is pretty close. According to the spec sheet it can handle up to 350w.

I have a NanoVNA and used that already successfully to tune my Sirio dipole (SD-FM DIPOLE 87-194 MHz).

Theoretically, it shouldn't be a problem - the yagi even has a gamma match. I read that tuning yagis can be a pain in the a** but apart from that my plan should work, right?

[3metrejim]

My first instinct would be to change all dimensions, multiplying by 83/93 (0.892). My second would be to build one seeing as the dimensions are given in the spec-sheet. I'd Then I'd look into seeing if the reflector could be replaced by the top of the mast to make construction easier (vertical polarisation only). Maybe try in some antenna simulation software first, before buying and trimming.

[BlackBeard]

My first instinct would be to change all dimensions, multiplying by 83/93 (0.892). My second would be to build one seeing as the dimensions are given in the spec-sheet. I'd Then I'd look into seeing if the reflector could be replaced by the top of the mast to make construction easier (vertical polarisation only). Maybe try in some antenna simulation software first, before buying and trimming.

Thank you! Why multiplying by 83/93? Trying the antenna simulation software seems to be a good idea. As far as I can see, it is possible to move the antenna's elements (reflector, dipole, director), so this does not limit you to tune the antenna. While it is true that I could build one myself, I'd like to stick to that one, as I made a good experience with the other Sirio antenna I have.

As for tuning, I'm unsure if I should use the values from those formulas (see photo) from here: viewtopic.php?p=38803&hilit=Yagi#p38803 or this calculator: https://sites.google.com/view/kn9b/yagi

Both were proposed on this forum.

[mikroman]

note that that plastic insulator will not survive long term operation above 200w. it will start to melt. I have one of their gamma dipoles.

[BlackBeard]

note that that plastic insulator will not survive long term operation above 200w. it will start to melt. I have one of their gamma dipoles.

Thanks for that hint! I operated the Sirio FM Dipole on 15w for a long time - always only 12h at a time though, I only broadcast occasionally. My new transmitter is set to 100w, it can output up to 150w. The dipole can handle up to 300w according to it's spec sheet but maybe that's a little unrealistic. For how long at a time did you use the dipole with 200w of power?

According to the spec sheet of the yagi, it can handle up to 350w CW(?).

[mikroman]

200w 0/24h works fine. Above that, especially in the summer months, problems arise. If you are in an area with high summer temperatures, I would not advise more than 150w. The gamma match construction of the yagi antenna is of the same material as the dipole. Check the nut connecting the gamma part and the connector from time to time. It can rust. Also expect problems caused by ice on the gamma match section.

I also tried their GP antenna (advertised "broadband", in reality with vswr from 1.25 to 3:1 across the band. Also has radials that fly into the sky pushed by slightly faster wind). I had also SIRO 400 MHz yagi through which water always finds its way to critical parts. In short, a cheap product that gives you a headache later.

[Albert H]

That's why I always made my own aerials!

[3metrejim]

Why multiplying by 83/93?

I must apologise, that was a typo. I meant 83/91 (0.912).... It's a linear size scaling from the original centre frequency (83 MHz) to the new centre frequency (91 MHz), a scaling of 0.912. Scaling all of the dimensions is a usual starting point for modifying an existing antenna, as wavelength goes down (frequency up) the size should shrink in direct proportion, it should perform the same at the new frequency.

Use the nanoVNA to check the performance matches the data before modifying, then try scaling of 0.95 first and make sure the swr response has shifted higher in frequency but still the same basic shape as in the original data. Take it slowly so you don't cut off too much element in one go. It's probably almost as fiddly as making one from scratch.

It probably won't come as much of a shock to you, but most 'pirate' antennas for 3m started off as amateur radio aerials that were scaled from the 2m amateur band. My home built co-linear was from the RSGB vhf/uhf manual (4th ed - pirate bible of the early 90's) and worked really well (The metal for it was also from sirio - an old CB antenna bought for £5 because some parts were missing).

[BlackBeard]

Why multiplying by 83/93?

I must apologise, that was a typo. I meant 83/91 (0.912).... It's a linear size scaling from the original centre frequency (83 MHz) to the new centre frequency (91 MHz), a scaling of 0.912. Scaling all of the dimensions is a usual starting point for modifying an existing antenna, as wavelength goes down (frequency up) the size should shrink in direct proportion, it should perform the same at the new frequency.

Use the nanoVNA to check the performance matches the data before modifying, then try scaling of 0.95 first and make sure the swr response has shifted higher in frequency but still the same basic shape as in the original data. Take it slowly so you don't cut off too much element in one go. It's probably almost as fiddly as making one from scratch.

It probably won't come as much of a shock to you, but most 'pirate' antennas for 3m started off as amateur radio aerials that were scaled from the 2m amateur band. My home built co-linear was from the RSGB vhf/uhf manual (4th ed - pirate bible of the early 90's) and worked really well (The metal for it was also from sirio - an old CB antenna bought for £5 because some parts were missing).

Thanks again! I thought so, 83/91 makes sense. I know that most pirate antennas started off as amateur radio aerials, that's why I thought it wouldn't be a problem to scale down a 2m antenna. But now I'm really thinking about building one myself because it's much cheaper and probably about the same amount of work, like you said.

I actually found the RSGB manual online but only the 3rd edition. If someone has a good instruction for building a 3-element-yagi, please send it to me

The only thing that makes me think is the gamma match. I saw those coax-loop gamma matches but they're probably annoying to match.

[3metrejim]

If you take a look in the crucial links section, I posted a link for an antenna book some time ago. It was still working a few days ago.
I think there are details of Yagi's in there along with useful charts for gain, and element spacing measured in fractions of wavelengths.
The RSGB seem to go all out on preventing book piracy online, the ARRL not so much.

[radium98]

you can upload the book here ?

[3metrejim]

you can upload the book here ?

A link to it, is on here. I try not to upload copyrighted stuff if possible.

viewtopic.php?t=2042

[radium98]

3metrejim thanks