[Johnson] I'm not an antenna expert

[joel hallas]

John,

I did a quick EZNEC pass at such a loop (in the absense of info I used a 
square) and, as you would expect, the impedance swings rapidly about 
with frequency, starting at >100:1 (wrt 400 ohms) at 160 and generally 
decreasing with frequency to a more reasonable 10:1 at the top of 10 
meters.

You didn't say how much ladder line (most I've used are actually 400 
ohm, in spite of what the package says) you are using from the tuner to 
the antenna, however that will transform the impedance depending on the 
length. A program such as TLW (comes with the current ARRL Antenna 
Book), or a Smith chart can be used to determine the actual impeadance 
at the bottom of the ladder line, if you know the length and have a 
measure of the loop Z (from a antenna analyzer or EZNEC).

Before I cut the antenna, I would try changing the length of the ladder 
line. The SWR will stay the same on the 400 ohm line, but the impedance 
the tuner sees will be significantly different. I'm almost certain that 
a short ladditional length, perhaps 6 to 10 feet  will move the Z around 
to where your tuner can handle it on 10 and 15 meters, although it may 
foul it up on other bands. You may find a length that will work for all 
bands - it's a crap shoot, or worst case you may need a few sections you 
can slip in when you change bands. I find the "double bananna" plugs 
(3/4" spacing) are handy for this function. Make sure the lengths are 
not coiled up or up against lossy material. In my celler shack, I've 
tacked such "delay lines" back and forth in the overhead floor joists 
with good results.

My predecessor at QST, Stu Cowen, N1SC, used a triangular loop about 
that size with good results for world wide coverage and found it worked 
a lot better after he tied a couple of fiberglass poles to his fence to 
raise it up. He also started at the second floor, may make quite a 
difference in low angle coverage, but it will still do well for NVIS on 
80 and 40 meters.

GL and 73, Joel, W1ZR

John Lawson wrote:

>
> I don't even play one on TeeVee...
>
>   So, in advance of a lot of EZNec work (and I don't have the 
> experience with that program to derive much 'good' out of it right 
> now) - I'd like to ask what may seem to be a rather obvious HF antenna 
> question.
>
>   Due to the usual reasons - my HF antenna is a loop suspended from my 
> backyard fence - approx 430' total, closed loop, 5' off the ground, 
> fed by 450 ohm ladder line back into the shack, using an Ameritron 
> ATR-15 tuner to match the system to my Valiant.  The tuner and 
> transmitter are bonded to a very heavy ground system via an 8' stake 
> less than 3' from the gear. There is no ground system under the 
> antenna, other than that which Nature provided - and with the current 
> winter conditions, the ground is rather wet and conductive.
>
>   This antenna system exhibits the following SWR:
>
> 160M - 1.3:1
>  80M - 1.1:1
>  40M -  +3:1
>  20M -   2:1
>  15M -  +3:1
>  10M -  +3:1
>
>  The tuner capacitors end up being  all-the-way-meshed on the 
> 'misbehaving' bands - not so on 160, 80, and 20.
>
>
>   So I'll see Y'all on 3880 and just fergit the rest.   ;}
>
>
>   No but seriously folks: obviously the feedpoint resistance is 
> outside the tuner's ability to cope with it at various frequencies.
>
>   I'm thinking the first unscientific experiment might be to go to the 
> opposite side of the loop from the feedpoint and cut it into a big 
> horizontal bent dipole - mainly because that will take about 45 
> seconds to accomplish - one of the benefits of having one's entire 
> antenna at ahoulder-height.
>
>   But I'd like to get some other opinions - I know there's an 
> electrical Pattern here from the info - and I have some other ideas 
> based on that.
>
>   And no, I can't put up a "real antenna" so I'm pretty much comiited 
> to making this one work as well as I can. Until I move the QTH to 
> somewhere with a few acres and room for Lots of Wire.
>
>
>   Cheers
>
> John  KB6SCO
>
>
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