[TowerTalk] antenna impedence and ground impedence??

[Dan Sawyer]

Tom,

Thank you for your reply. Yes, ground resistance.

This antenna is a custom 'screwdriver'. The base is about 4 inch 
diameter 1/8 inch wall aluminum tubing. The coil is wound on a 3.5 inch 
form of number 12 solid wire, the pitch is about .5. The connection 
between the tube and the coil is made from a ring of copper 'fingers' 
that make a good contact. The top is a 6 foot 1/8 inch solid steel rod. 
Models of this produce between 1.5 and 2 ohms radiation resistance.

That said the feed point resistance consistent measures about 56 Ohms at 
the antenna with an Autek RF1. The antenna matchs w/o a tuner at about 
1.2 at the end of about 100 feet of RG8. The cable tests clean with a 
TDR all the way out.

The objectives are to learn a good approximation what is actually going 
on and to optimize the system performance.

Thanks,
Dan Sawyer



Tom Rauch wrote:

>>I have a questions regarding calculating ground impedance. How can this
>>be measured??
>>    
>>
>
>I think you are asking about ground resistance, rather than impedance.
>Measuring ground resistance can be very difficult unless you know every
>other variable involved, including everything about the effects of
>surroundings.
>
>  
>
>>The impedance of a loaded vertical measures about 50 ohms, however model
>>programs predict the antenna impedance is in the range of 10 ohms. Does
>>this imply ground impedance is 40 Ohms??
>>    
>>
>
>No. But it does indicate you have something seriously wrong with the
>antenna, the model, or a combination of both.
>
>I assume you have a loading coil. It is not possible for most people to
>measure loss resistance in a loading coil, so you don't know that value.
>Even if you knew the loading coil ESR, you would have to normalize the loss
>resistance of the coil to the feedpoint of the antenna since that is the
>reference point of the measurement.
>
>It is also very difficult to measure actual radiation resistance.
>
>Radiation resistance, at least by any meaningful use of the term, is NOT
>the real part of the feedpoint impedance. Ground loss resistance is not the
>difference between the radiation resistance and feedpoint resistance except
>under very specific conditions.
>
>What is the antenna?
>
>73 Tom
>
>  
>