Yagi for Channel 4?

[chamcooker]

Ive been lurking for a while on this forum and reading a lot ,understanding some and ceartainly learning a lot. An empty vessel is easy to fill and all that.
My specific problem is that the channel I really want is cbc on channel 4 ,60 miles away(70mhz if I have it right) .Hockey ,you know. Using k7mem's quick calculator I found I would have to have a boom length of something like 24 feet.Now I could mount that along the ridge of the house , only it runs the wrong direction! The calculator kept admonishing me that too short a boom would not give a high enough gain. That discouraged me. Now I see the elegant work posted by Mlord and Im inspired to try some thing. . Im wondering if I might get ok results with what looks to me to be compromise designs posted in other threads by the meteor watchers.(sky scan scienc awareness project)with boom lengths of .45 or .65 the wave length.(4.28m).Im also wondering if 1/2 in cu pipe from the scrap yard would work using the common sweat solder fittings or is it too big in diameter

[300ohm]

The calculator kept admonishing me that too short a boom would not give a high enough gain. That discouraged me.

Whats your dBm for that channel per TVFool ? VHF-Lo channels from long distances can get by with lower gain antennas.

Im also wondering if 1/2 in cu pipe from the scrap yard would work using the common sweat solder fittings or is it too big in diameter

1/2 in cu pipe is fine for VHF-Lo channels. In fact thats the size used most commonly in Australia for VHF antennas. Parrot problem, heh.

[chamcooker]

300 ohm,the nm(dBm) for this channel is 5.3.I get it badly with rabbit ears.think a yaggi with the longest boom length thats practical is worth trying? the 1/2 in copper might be good in freezing rain as well as parrots.

[300ohm]

think a yaggi with the longest boom length thats practical is worth trying?

Yep. I would say a yagi with one reflector and 3 or more directors would do the job for a vhf low station at 5.3 NM.

the 1/2 in copper might be good in freezing rain as well as parrots.

For freezing or any rain, you want to make sure the water has a way to escape out of the pipes.

[mr weather]

If you can build it from aluminum tubing it will be a LOT lighter in weight.

[chamcooker]

certainly the weight is a factor but cost is too .so if scrap copper pipe is available I'll use that to experiment.Im thinking of a boom made of abs plastic and copper for the elements.can anyone contribute some design detail ideas?eg do the ends of passive (?) elemnts have to be capped ?

[stampeder]

I would not use ABS for a boom. If you go to a hardware store, grab a 10 or 12 foot length of ABS pipe and hold it out like a sword and give it a shake and you'll see why - its not as rigid as you expect it to be.

[mlord]

The boom will have to be something stiffer than ABS, unless you're keeping it really short (and/or really large diameter). ABS bounces (flexes) a lot. And for VHF-4, the elements will be long (and therefore heavy), so something stiffer may be necessary for the boom.

[mlord]

do the ends of passive (?) elemnts have to be capped ?

No. But I do notice that they are squished into tips on all of the commercial antennas I have here. Dunno why, unless it's to slightly stiffen them.

I suppose closing the ends (or just plugging them with hot-melt glue) does prevent ice buildup on the insides. Dunno if that's a real problem or not, though.

[300ohm]

Squishing the ends also stops whistling. As far as keeping water out, it may be a good idea, but a lot of the commercial antennas Ive seen have the seam end facing up, not down, on a lot of the elements. Maybe thats an error of poor quality control ?

[chamcooker]

whistling huh,so thats whats meant by tuning an antenna!yu,yuk. Ill flatten the ends . Ill look for something stiffer than abs perhaps thick walled plastic pipe. I'm presuming that if the boom is plastic i can run the dipole element right through it the same as the elements. Is that right?

[300ohm]

Yeah, if its the common white pvc plumbing or gray electrical pipe (which is less $ and at least partly UV resistant). Just make sure water can get out.