[TowerTalk] End feeding a half-wave vetical

[K4SAV]

Jim Smith wrote:

>
>It did occur to me that I could add a 1/4 wave of wire to transform the  
>impedance down to something easy to match so I modelled a 3/4 wave 
>vert.   Oooohhh.... yuckkk..  Not a low angle antenna.
>
<snip>

Yep, a 3/4 wave antenna is not a low angle radiator.  Anything longer 
than 5/8 wave and the maximum lobe starts to go up.  That's the reason 
you see so many 5/8 wave mobile 2M antennas.  You may have intended your 
1/4 wave transformer to be similar to the use of a stub, which you 
mentioned next.

<snip>

>
>Another, who has an array of 1/2 wave verts on 160, pointed out the 
>difficulties  in using stubs when he said, "I did look at using a 
>quarter wave coaxial stub to  lower the feed impedance to something 
>manageable. Problem with that was that  even with 7/8" hard-line at 160m 
>the loss was some 10dB. Good match though!!!!"
>
<snip>

Yep, you have a piece of coax operating at extreme SWR.  Losses are 
going to be very high.

<snip>

>
>Another suggested a 1/4 wave xfmr using 450 ohm ladder line.  I had 
>thought of  this but it seems to me that the currents in the line would 
>be very unbalanced,  leading to messing up of the pattern due to high 
>angle radiation from the line.   This assumes that the line is running 
>parallel to the ground and not very high  above it.  If the line is 
>running vertical and connects to the 1/2 wave vert 1/4  wave above 
>ground it seems to me to be the same situation as with just a 1/4  wave 
>length of wire, given the current unbalance in the ladder line.
>
<snip>

Actually the currents aren't significantly unbalanced.  This is where 
you are reinventing the J pole. The unterminated side of the 450 ohm 
line acts as the matching network.  Its radiated field is cancelled by 
the field produced by other side of the 450 ohm line that is attached to 
the 1/2 wave antenna. So the antenna system performs like a 1/2 wave 
antenna, end fed with a matching network that should radiate very 
little.  It doesn't matter if the 450 ohm line is vertical or 
horizontal, as long as it is not too close to the ground.  Same rules 
apply as to using open line, you can't put it in close proximity to 
other conductive objects. You will also need another matching network to 
match the 450 ohm line to 50 ohm coax.  This network should be much 
easier to design, however.  Or you could just do the same as is done 
with a J pole.  You can make this work without ground radials, but it 
won't be as efficient as a 1/4 wave antenna using a good ground system. 
 If you can't implement some kind of radial system (either in-ground or 
elevated) this may be your best choice.

<snip>

>
>Another proposed a 3/8 wave vert as being easier to feed and working 
>just FB  (310 countries on 75 from Ct).  My model says - 3 dBi at a T/O 
>angle of 21 deg  which is 4 degrees worse than the 1/2 wave.
>
<snip>

Yes 3/8 wave is easier to match than a half wave. You will need ground 
radials for this. Performance is not much different than a 1/4 wave 
antenna using ground radials.  (Note: Working countries is not a good 
antenna test. As a kid I worked a bunch of countries with a piece of 
wire laying on the floor in my room, but this doesn't make it a good 
antenna.)  Although, I will admit that a 3/8 wave antenna with a good 
ground system, is a good antenna for 80 or 160.

<snip>

>
>I'm going to have to play with NECWin+ some more to make sure I actually 
>know how to use it (given that I seem to be disagreeing with ON4UN) 
>before I pursue this any further.
>
<snip>

Good idea.  Vertical performance is very confusing, since most modeling 
programs don't do a very good job (it's a difficult task).  Seems like 
they are all slightly different, and I am not familiar with NECWin+. 
 NEC4 seems to be the best, but not many people have access to that. 
 EZNEC gives you several choices of ground types which works OK, if you 
understand the limitations. Check the stuff in ON4UN's book (most was 
done with NEC4) and if you have a big disagreement, then you need to 
resolve it.  Remember that even the best of these programs, have 
somewhat iffy results when modeling vertical antennas.  Most of these 
programs are "good enough" for horizontal antennas because the ground 
isn't being used to obtain the return currents for the antenna, and the 
losses, in general are lower (unless you have a very low horizontal 
antenna).

Jerry, K4SAV