Dual Band Yagi-uda for 2m/440

[RC286]

Hey everyone.

I have an old TV UHF yagi, roughly 10ft boom. and am looking to re-purpose it as a dual band antenna for 2m/440.

I am planning on making a folded dipole arrangement, and although I know how to calculate the spacing and lengths of the parasitic elements, I can't seem to find out how to properly space the driven elements from one another.

Some designs I have seen have both driven elements one atop another on either side of the boom. I have also seen designs where the driven elements are one in front of the other, usually the larger DE (in this case 2m) slightly behind the smaller one (440) What I can't figure out is how to calculate this spacing. Does anyone have any ideas or links to somewhere that explains this.

 

[K7MEM]

You could certainly build it so that the 2M and 440 elements are interleaved. On 2 Meters the 440 elements will be too short and not be active. On 440, the 2M element will be too long to cause much problem.

If I were to re-purpose a TV antenna for dual band use, I would build it so that one antenna (440) is in front of the other antenna (2M). Or with the 440 elements at the very front where there will be minimum interaction with the 2 Meter director elements. With this arrangement, the elements for each band will be well out of the way of each other. On a normal TV antenna, it is usually constructed so that the UHF section is in front of the VHF section.

A TV antenna is usually built as a Log-Periodic. So the longer elements, which you would want for 2 Meters, get short very quickly. Often times they rotate the element ends forward. This is intended to smooth the transition of the active areas. What this all means is that, you will have to change the element mounting and that a lot of the elements will not be useful on 2 Meters.

Antenna gain is directly related to boom length. With a 10 foot boom, only seven elements (1-Reflector, 1-Driven Element, and 5-Directors) for 2 Meters will fit. This will give you a maximum gain of around 10 dBd. But that's still pretty good.

To get an idea of element lengths and spacing, try this link below. It gives you a lot of room to adjust to your requirement. Just do two designs. One for 440 and another for 2 Meters, and then add them onto the same boom.

Martin E. Meserve - K7MEM - VHF/UHF Yagi Antenna Design

There is also a link available for creating the folded dipole feed and matching it to 75/50 ohm coax.

Martin - K7MEM

 

[gewecke]

Try looking at the design measurements for a Cushcraft A-270-10s

Which (I think) is the model number for their dual band yagi ?
I used to have one up years ago, and left it when I moved. Now, I wish I'd kept it.

73,
n9zas