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Discussion in 'Amateur Radio Antennas' started by eacky, Nov 4, 2011.
SY27-4 cb yagi to 10m.
One method is to measure each element. Then multiply those measurements by the antenna's resonant frequency (where it's tuned for now). Then divide those numbers by the new frequency you want the thing to be resonant on. Convert those feet and inches to a decimal number (18 feet and 3 inches would then be 18.25 feet). Then divide that number by the frequency. Keep the frequency in the same measurements too. Channel 19 is 27.185 Mhz, so the new frequency should be in Mhz too.
That should get you in the ball-park, and then using the gamma match to do the impedance matching just like you did for 11 meters.
*Tune Gamma match for ~27.555mhz
*Use antenna tuner to get 10m
*That is one way - but it does work
Go through the painstaking process of trimming each element length by dividing 27.205mHz (stock tuning for this beam) with the frequency center that you want to tune it to - say 28.400mHz phone band. I get ~0.957922; this is what you use as a multiplier for each element length. The spacing between the elements will also need to be shifted by that same multiplier.
But I use method #1.
Although it might not be the best/proper way and it will change the lobe pattern somewhat, it works without much fuss.
scroll down the page and you will find dimensions for a 10 meter yagi.
Soooo . . . . Having said that, the way to change the beam for 10m is to scale it using the formula above for the element and spacing between the elements. That should work out OK.
This Sirio beam beam was 'computer optimized'. Which I suspect means that some engineers at Sirio used some EZNEC program to make it work on a short boom length. This part is speculation; but if you must you might try to contact Sirio and ask for the details. If you must. I did.
That is the same info the Mr Mezzadrelli sent me concerning re-tuning the beam for a different freq center. But I would double check my math to be sure that it is correct before you commit to cut or change anything.
Ing. Mezzadrelli Lorenzo
SIRIO Antenne SRL
Via Liguria 15
46049 Volta Mantovana (MN) - ITALY
Ph. +39-0376-801515; fax +39-0376-801254
Sirio Antenne - High Performance Antenna
Optimizing one particular aspect/characteristic of a directional antenna is always a compromise with it's other characteristics. If you want some of that 'optimizing', then go get it. That means using a tool that can tell you how to go about it, and that generally means an antenna modeling or 'optimizing' program. There's quite a few of them floating around (and for a long time) so finding one shouldn't be all that difficult.
Sent to me from Sirio/Italy:
Dear Mr. Xxxxxx:
SY 27-4 is not a tunable antenna and SIRIO ANTENNE doesn't effect
substitutions and doesn't guarantee the correct working of altered products.
However, if you really want to try to change the work freq. of the antenna,
I can give you a method.
To work at 27.555MHz you must to RESCALE all lengths using this formula:
Final_length = (27.2 / 27.555) x Initial_length = 0.9871 x Initial_length
REFLECTOR : New length = 0.9871 x 5740 = 5666 mm
DRIVEN: New length = 0.9871 x 5400 = 5330 mm
1 DIRECTOR: New length = 0.9871 x 5130 = 5064mm
2 DIRECTOR: New length = 0.9871 x 4890 = 4827 mm
Initially, I suggest to not rescale spacing between elements (only for
possible problems) and verify the correct electrical working.
After the rescale you must to tune the antenna moving the gamma-match and
using a ros-meter (I can't help you now; the new position is not prevedible)
If the antenna doesn't work well; then you can also try to rescale the spacing
between elements (using the same formula above)
Reflector-driven: New length = 0.9871 x 1570 = 1550 mm
Driven-1 director: New length = 0.9871 x 1020 = 1007 mm
Director 1 - director 2: New length = 0.9871 x 1410 = 1392 mm
Then re-tune the antenna by moving the gamma-match.
I remark that this is only a theoretical method and we don't guarantee the
correct working of antenna.
Ing. Mezzadrelli Lorenzo