Very narrow sweet spot yagi

[bigloudvtwin]

Well I built a 5 element yagi today for 70 cm. Used an online calculator for lengths , spacing etc. I was shooting for 435mhz. Well according to my rig, the sweet spot is about 436mhz. That's ok BUT, at 430on down and 440 on up swr shoots through the roof and my rig (ft857d) starts flashing me. I made a gamma match for it that is approx. 6" long. Maybe it's too long. Seems like the antenna should have a wider freq range.

 

[W5LZ]

I've only dealt with one 70 cm yagi and it's elements were something like 13" long. So, I have to think you're right, a gamma match 6" long might be a bit large. The one on that one yagi I have dealt with was only something like 2" maybe.
I think the gamma is responsible for the bandwidth of you antenna to some extent, but I also think you shouldn't count on the yagi covering the whole 70 cm band, it won't.
- 'Doc

 

[WA8ZYT]

Check out the antennas at g0ksc.co.uk. The LFA yogis are direct fed, no matching device needed. And they are wide band, to boot.

 

[W5LZ]

This loop fed antenna isn't a 'new' thingy at all. I think Mosely(?) used a folded dipole as a driven element, which is just an extreme in the shape of a loop. And since each additional element to a yagi reduces the input impedance of that yagi, manipulating the shape of that loop will certainly affect the input impedance, make it closer to the desired 50 ohms.
None of this is to say that those 'LFA' antennas don't work well, they evidently do. It does say that this isn't an original concept...
- 'Doc

That 'broad bandedness is always relative to frequency of use. At 400 Mhz, never expect much 'broad bandedness', especially if the band is as wide as 70 cm. That 'broad bandedness' is -always- at the expense of efficiency, or 'Q'.

 

[WA8ZYT]

I have built 2 LFA's for 6 meters, and one for 2 meters. All 3 antennas work very well, and I am completely satisfied with them. You can home-brew one for about 1/4 the cost of buying one. Actually, I built the 2 meter 9 element with leftovers from the 6 meter project, except for the boom and boom to mast clamp.

 

[W5LZ]

Good! I don't do enough 'directional' stuff on 2 meters to make an beam antenna worth while. Oh man, it isn't the beam that costs, it's the rest of what's needed to get that beam in the air and turn it, you know?
- 'Doc