Topband: T-antenna

[Joe Subich, K4IK]

David, 

> the first antenna has a radiation resistance of 10 ohms while 
> the flat top is 8 ohms. What I don't know is what the 
> equivalent ground loss resistance is. I guesstimate 25 ohms. 
> If so, the difference between the two antennas is 0.7 dB. 
> With a 50 ohm ground the difference is 0.8 dB. With a 12.5 
> ohm ground the difference is 0.6 dB. To me it looks like a 
> wash- I would use the flat top rather than put the inverted-l 
> back up. If one dB or so matters to you (and it does to the 
> big guns) then the inverted-L might be worth the effort. 

There are several considerations for a T antenna vs. inverted L. 

1) the horizontal section of a T antenna is about 50% longer 
   than the horizontal section for an inverted L when the 
   vertical section is the same length.  If one has limited 
   space or must mount the vertical section at one end of a 
   space, the inverted L is indicated. 

2) The inverted L has a slightly higher feed impedance (as 
   you indicate). This will result in marginally higher 
   efficiency for the same ground system.  However, if an 
   inverted L is mounted at one edge of a space and the 
   ground system is not uniform, moving the vertical section 
   to the center of an area might provide the opportunity 
   to install a more efficient ground system. 

3) The inverted L has a slight degree of pattern asymmetry 
   about 2 dB in the direction opposite the horizontal wire. 
   To some people, that may be significant. 

4) The inverted L has a high angle, horizontally polarized 
   lobe.  Depending on height of the vertical element, the 
   high angle lobe can be as strong as the low angle lobe 
   ... and will certainly hear more close in QRM/QRN. 

73, 

   ... Joe, K4IK