[TenTec] OT: Indoor antenna

[Dr. Gerald N. Johnson]

There have been such tables since antennas began. Certain lengths of the 
top and the feed are easier to feed being closer to resonance at a high 
or low impedance point at the transmitter end of the feed. Often the 
feed plus half the flat top length comes close to any multiple of a 
quarter wave. When an even multiple the feed Z is high, when an odd 
multiple the feed Z is low.

There is another school of thought that when those preferred lengths 
don't fit the premises or the wire on hand, is that the preferred length 
is the length that fits between the supports and the preferred feed line 
length is the length that fits from the antenna to the tuner in the 
shack. This can demand the tuner handle more obnoxious loads. This 
school believes tough tuning and being on the air is better than being 
quite bemoaning the preferred antenna won't fit the space available.

I notice in MFJ automatic tuner manuals that they don't like antennas 
that present a high impedance resonant condition at the tuner. Which 
means their tuners may be limited on voltage handling capability.

When a wire gets to be several wavelengths long or longer, the effects 
of a resonant length are much smaller because the part near the feed 
radiates and there is less current at the open end to be reflected. The 
wire acts like a traveling wave antenna rather than a standing wave 
antenna. Many computation techniques, from days of old and computers 
neglect that change in current from radiation and give erroneous results 
on long wire antennas. In feed Z and Z vs frequency and the radiation 
pattern.

But one might need to be more careful about the length and orientation 
of a long wire if one wishes to work in a certain direction. Like Europe 
from the USA because there are more DXCC countries in that direction. 
Then its important to use a NEC based antenna analyzer to learn the true 
current distribution and radiation pattern.

Otherwise what you can put up will allow making contacts, which is often 
a better situation that not being on the air for lack of an antenna.

73, Jerry, K0CQ

On 11/29/2010 7:29 AM, kf6e at mail.com wrote:
> Here's a link to an article listing the best lengths to use for a random wire antenna:
>
>
>   http://www.hamuniverse.com/randomwireantennalengths.html
>
> I didn't measure mine; I just strung up some wire.  I suspect you will have better results if you use one of the lengths suggested in the article.
>
>
>
> 73,
> Frank
> KF6E
>
>
>