[Antennas] Re: antenna height on 15/17 meters

2004年3月25日 11:13:17 -0600

Well, you asked :)
Antenna height is "electrical" height vs physical height.
Although they are related, your ground conductivity comes into play. As an
antenna is raised beyond the 1/2 wave you mentioned, several things happen.
One of them is the introduction of multiple lobes as various angles of
radiation. If you were to take a look at an antenna book this should be
confirmed. In other words, what you expect is not necessarily what you get.
(for the average person)
If it were me, I would simply put an antenna up a high as practical
considering your budget and neighborhood. That is unless you are into
measuring ground conductivity in both the near and far fields. Something to
remember..... Who do you want to talk to?
Keep in mind that some of the top contesters use lower antennas (even fixed
directions) for spotting and a "biggie" for breaking through the crud. If
you want to talk primarily stateside, a lower antenna is a better bet due to
higher angle of radiation. If you want DX, go for all of the height that
you can muster.
Do not place your antenna at the same height as power lines etc. This will
tend to cause reduced apparent F/B ratio and reduced apparent gain. The
reason for this is that those power lines tend to act like antennas
themselves. If the power lines were north of you for example, and you had a
yagi aimed north (at the same height) you would most likely find that you
were hearing stuff from the backside of the antenna stronger than you would
if you were not at the same height as power lines.
You will also find that yagi or any other type of gain antenna will still
work at reduced heights better than a dipole at the same height. In other
words, if you can't get a super height, go ahead and put the antenna up.
While the apparent gain might be reduced, you will still have gain over a
simple antenna.
I know of a well know contester that even had place a yagi at 20 feet. He
used this antenna on 15 meters. This lower height was used simply because
of the area that he was living in. Later of course, he went to 200 foot
towers.
Also, and of course, please stay away from power lines and use the proper
safety precautions. <-- that is my disclaimer :)
Hope this helps...
73 es GL
Steve
> Message: 4
> Date: 2004年3月24日 19:55:34 -0800 (PST)
> From: John Geiger <[email protected]>
> To: [email protected], [email protected],
> [email protected]
> Subject: [Antennas] Antenna Height for 12 or 15 meters
>> Maybe someone on the reflector has antenna modeling
> software and can help me, or maybe they just already
> know the answer.
>> Here is my question. I have thinking of going to a
> monoband yagi for either 12 meter or 15 meters.
> Really like those 2 bands, and yagis aren't too large
> for those frequencies. It would probably be mounted
> on a tripod on the roof of the house, which is where
> the current antenna is. This would get it about 25
> feet or so above ground, which is over a half
> wavelength for both 12 and 15 meters.
>> It seems to me that 1/2 wavelength is the magic height
> to get a yagi to perform. Is that correct? How much
> better would it perform by going to 40 or 50 feet?
> Would it adequately perform at 25 feet, having a
> decent angle of radiation and such?
>> 73s John NE0P