How high MUST an aerial be?

[The squire]

Hi

I was just wondering is there a minimum height an aerial MUST be off the ground? I was running my BW tx through a Sigma SDH100 for about 6 months, just from the studio. It was about 15 feet off the ground and it worked fine!

I was testing it from a far higher location recently, but it was just a rushed 15min job with no mast to use, so i had the bottom of the dipole about a foot from the ground.

When tuning in from a car radio, it wasnt really picking it up, but what it was picking up was like a whirring, clicking sound!

any ideas?

[Ben Murray]

I remember being told by an engineer that the closer it is to the ground it causes the signal to bounce back at the aerial, probably whats causing that whirring sound.

From a site you'd be ok with a 30-40ft mast, don't know about from anywhere else though.

[The squire]

That might explain it! thanks!

[Mike]

The minimum recommended is 30ft (ideally you want a good bit more -even at a hilltop site)

The most obvious reason is so your signal isint blocked by nearby obstructions (trees/buildings etc)

Another reason is because ground reflections can mess up the directional pattern (and in severe cases even the SWR and impedance) of an aerial if its too close to the ground (what Ben said)

Unless youre only doing a quick test Its an act of madness transmitting from an aerial 1 foot (or even 1 metre) above ground and for safety reasons (someone touching the thing and getting an rf burn or the aerial accidently shorting to ground) Id never run more than a couple of watts into such a setup

[The squire]

Oh yeah, I know. I was just testin it from a new location and i only had bout 10 mins to set up, test, and take apart the rig!!