[OZAPRS] HF antenna

Terry Neumann tfn at rbe.net.au
Sun Jul 3 21:57:56 EST 2005


Andrew Rich wrote:


Gents,



I calculated an inverted V as 10.147.600 = 6.677M (21.905 ft) per leg does

this check out ?



Signals are there just weak

  


Hmmmm...   It could be a bit short on the ends Andrew.

By my calculation that length should work out to be centred around 11.23
Mhz ....    

I use the following formula as a starting point for antenna construction:

Full wavelength (metres)  = 300 / Mhz.

The result needs to be divided by 4 to get 1/4 wavelength  - each leg of a
dipole or inverted V - or divided by 2 to get the complete dipole length.
In general, the calculated length will be similar for either a straight
dipole, or a dipole in inverted V configuration.    

Using the above a suggested starting point for (say) 10.125 would be
(300/10.125)/4  =  7.41 metres for each leg.   In practise, other factors
will have an effect on the final length of any antenna installation -
things like the proximity of other antennas,  and the tower or support for
the antenna can all affect the end result.    

It's suggested that one should start with (say) 7.6  or 7.7 metres for
each leg and then use the SWR meter to test for the best match, and
shortening the ends of the antenna to get things down to where you want
them.   I wrap the unwanted length past the end of the insulator back over
the remaining active end of the dipole rather than cut bits off the end.
It's easier than rejoining bits on again if you go too far and take too
much off. (guess how I discovered this.....)  :-( 

This is probably most unscientific, and I could be wrong, but  it works
for me.

BTW Bill Orr and Stewart Cowan in Wire Antennas for Radio Amateurs use the
following formula for a dipole:

Overall Length (feet)  = 476/Frequency (Mhz).   This gives a more or less
similar result to the metric version.

They also suggest that the included angle at the apex of an inverted V
should be greater than 90 degrees.  

Great stuff - wire antennas!

FWIW

73
Terry
VK5ATN 







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