[ICOM] G5RV & AT-500

[joel hallas]

Al,

I'm not sure exactly what the AT-500 is, but would guess that it's an 
auto tuner. They are great, but one limitation is that on most, you 
can't tell what the settings are. The fact that it tunes to 1.35:1 
doesn't say much about the antenna impedance, since it will likely tune 
a wet string as well, and from the shack you can't tell which you've got.

The best diagnostic step I can think of, other than taking it down and 
measuring all connection resistances - as has been pointed out, it 
doesn't work well with one side disconnected, is to measure the SWR, or 
even better the impedance at the bottom on each band. I do that whenever 
I put up an antenna and then can tell if it has changed.

My G5RV, many times broken and reconnected, hence shorter -  has the 
following measured SWR (average across band) at the bottom of 100 ft RG-213:

80/75; 5:1
40; 2.7:1
30; 10:1
20; 2:1
17; 2:1
15; 7:1
12; 6:1
10; 3:1

Yours may well be quite different, depending on actual lengths, but it 
should be somewhat like this.

GL & 73, Joel
Joel Hallas, W1ZR




















Alan Leith wrote:

> I've finally raised by G5RV again, after it came down a few weeks ago 
> in an accident.  An old tree fell across one end and...well that's not 
> the problem.  Anyway, before it fell down, the antenna was performing 
> well on 12, 17, 20, 30, 40 and 80.  I tune the antenna with my AT-500 
> and SWR was below 1.5-1 on all those bands.
>
> When it went back up today, I was able to tune it on the same bands 
> EXCEPT 17M.  For some reason, while I can tune it down to 1.35-1 in 
> the pre-set mode, once I switch to auto tune, the SWR goes up to 3-1 
> and the power output is cut to 50W.
>
> As far as I know, nothing has changed, except that this time, I pulled 
> one end tighter (I had not done that the first time) and about 3 1/2 
> feet of the end is folded down over a tree branch.  Is it possible 
> that this has made an electrical difference in the length of one leg?  
> But why does it tune so well on all bands except 17, on which it 
> worked perfectly well before?  I'm not interested in using the antenna 
> on 10 or 15.
>
> Can I do anything to fix it?  It is the only antenna I can put up for 
> 17M and that's one of my favorite bands.
>
> Help.
>
> 73
>
> Al, VE1AL
>
>