Mounting a Yagi?

[Mike Alpha]

I recently acquired a 16 element UHF yagi. It's a RFI YB series yagi with directors welded to the boom.

The previous owner said he never had much luck with the antenna not really receiving the signal he was pointing it at. He had it mounted to the mast at the centre of the boom using a U-clamp. The 3 metre mast was on a base plate bolted to his clip-lock roof.

It made me think his lack of luck with the antenna may have been because the boom of the antenna was basically shorted to earth.

So I wondered what is the correct method to mount this type of beam antenna? TV antennas and other wifi type beams have the directors insulated from the boom and the mast usually connected at the centre. Why are the directors insulated from the boom in these type of Yagis?

The RF Industries catalogue shows all their Yagis mounted to the mast at the rear of the boom. If the mast is earthed will this cause reception issues? I've read that an earthed boom means less noise and better lightning protection. My mast is bolted to a brick wall so technically it is not earthed.

What do I do???

Mike

[citabria]

Hi Mike,

Remember that earthing is basically a DC only phenomenon, so it's pretty much only going to impact lightening and stray voltages/currents. Yes, having the mast in the middle of the thing most likely didn't help performance one little bit, but it's more to do with the mast interfering with the directors and being completely the wrong length for the position it was in

Having the mast earth couldn't hurt at all, think about the hundreds (thousands) of commercial installations that are on earthed towers.

Hook it all up and let us know how it goes...

Cheers,
Matt

[ivahri]

Mike,

YB series yagis are supposed to be END mounted- behind the reflector. The mast should be earthed because you don't want the earth return to be down your feeder if there is a lightning strike... You never ever clamp them in the centre because the mast is going to seriously impact on the radiation pattern (worst case it could turn the antenna 180 degrees) particularly if it is in the same plane as the elements. Where antennas need to be braced (YB16) we use non metallic braces...

Cheers

Richard

[system_tech]

Yeh, what Matt and Richard said.

If you really must support the antenna in the middle, you need to have say a metal mast with say a 2 metres plastic or conduit (only) section immediately prior to the antenna.

This is not recommended though, you miss out on the earthing as stated above.

As Matt said there are 1,000s of these out there, earthed, albeit at the mounting end. In my company, and many others, if we are not satisfied with the metal to metal earth path we will run a 16mm2 or 25mm2 earth cable to the antenna array. Sometimes larger cable.

Oh and try a bit of fun end mounting say a 6 element VHF yagi It certainly can be done.

S T

[Mike Alpha]

Thank you gentlemen. I will heed your advice when I mount it. I will have to run an earth strap to the mast now. All my earth returns are down the coax cables into my radios.

I became curious when I noticed some Yagis have insulated directors and others welded. Reading up on it further it seems it doesn't make much difference to performance, so as long as the welded director construction isn't interfered with at the forward end of the boom.

Mike