To: <towertalk@contesting.com>
>Date: Fri, 4 Sep 1998 13:07:28 -0700 (PDT)
>From: Pat Barthelow <aa6eg@cv.tmx.com>
>
>After fixing several bugs in the transmission line, and replacing
>the BN-86 Balun on our Hygain LP 1017, the antenna is still not
>performing (or, more correctly, matching) very well. SWR
>generally too high (4.0 to 1 at some freqs) but has the
>characteristic (log periodic) rise and fall of impedance across
>it's design range. (6.2 to 30Mhz). In some cases (frequencies)
>the SWR is acceptable, as low as 1.2 to one. The BN 86 balun was
>replaced with a coiled coax balun, carefully made, wound as a
>cylinder, not jumble wound.
I'm not familiar with this particular antenna. But I have
experience with a number of other LPs from Hygain and others. I
don't think a choke type balun is appropriate at the feedpoint of
these antennas. If they are correctly designed, the feedpoint
impedance will be in the 100 to 300 ohm range. Most are supplied
with some type of 4:1 balun to match 50 0hms.
LPs can be designed to match 50 ohms directly. But if they cover
a large frequency range, measures must be taken to prevent
parasitic modes from destroying performance at some frequencies
in the range.
>
>I am looking at the Delta (?) matching section. It consists of
>an aluminum conductor parallel to and centered on the largest
>element(s),at the back end of the boom, and which are fed at the
>end of the parallel feed of the LP. It is about 12 ft long, and
>hangs abt 8" below the big end (6.2 mHz end) LP element(s). It
>is attached to each element at it's ends, with aluminum strap. It
>was bent up slightly due to catching a nearby wire antenna, and I
>wonder if it's positioning or spacing is critical to matching the
>LP. Any ideas?
This sounds to me to be more likely a linear loading section than
a matching device. The only matching device I have seen at the
back end of LPs is a hairpin. Usually the hairpin will extend
parallel to the boom. Either straight back or folded back
against the boom if the hairpin is very long. If it is a linear
loading section and it was bent enough to cuse it to touch
anything it shouldn't, it could have a large effect at the low
freq end of the antenna's range.
73, Eric N7CL
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