RE: NEC-LIST:Proper Antenna Patterns

Ian,
        I believe I follow you example, but if I have a LP that is not matched to
50 ohms and I want to feed it in NEC with a 50 ohm source, how do I model a
50 ohm source in NEC connected to the feed segment and the internal
transmission line?

-----Original Message-----
From: Ian Roberts [mailto:ITR_at_nanoteq.com]
Sent: Thursday, October 11, 2001 9:31 AM
To: 'KLysiak'; nec-list_at_gweep.ca
Subject: RE: NEC-LIST:Proper Antenna Patterns

I have just gone through this exercise with a recently completed 14 element
LP for 48-90 MHz.
Use LPCAD27 at the unofficial NEC homepage to design the basic antenna using
an impedance of around 80-90 ohms for the LP transmission lines. This will
translate to about 50 ohm resistive with low reactance at the 50 ohm feeder.
Save to NEC format which will also define the transmission line in the NEC
file. Pass the NEC file through a NEC2D engine; a recommended user interface
is Arie's first class 4NEC2D at the same site, which also allows stacking of
polar plots to compare antennas, and also generates frequency sweeps if
defined in the NEC cards, etc. Use the default values in LPCAD27 (don't
select your own tau, etc values) which produce a design with a b/f ratio of
30 dB and gain of around 7-8 dBi for a standard LP boom length. Minimum
number of elements for decent performance is around 12.

Fine tune the transmission line impedance in the NEC file to get an exact
match. In the physical domain this is the spacing between the 2 "booms".
Return loss is per design, I obtained around 1.5 SWR across most of the
design bandwidth, with a peak of 1.9.

A simple 1:1 current balun rounds off the design details, this can
conveniently be the coax fed inside one of the LP booms to the feed end of
the antenna to keep RF off the coax's screen.

Cheers,
Ian ZS6BTE

-----Original Message-----
From: KLysiak [mailto:KLysiak_at_swri.edu]
Sent: 11 October 2001 03:37
To: nec-list_at_gweep.ca
Subject: NEC-LIST:Proper Antenna Patterns

Colleagues,
        The complex (real and imaginary parts or amplitude and phase)
antenna
patterns for transmit and receive antennas change with termination
impedance. Normally NEC uses an ideal conjugate matched source on transmit
antennas. If the antenna is fed with something other than an ideal conjugate
load (i.e. 50 ohm coax) its pattern will change.
        How can this be properly modeled in NEC? In particular, if we want
to model
a LPDA that has internal transmission lines which we want to feed with a 50
ohm source. How do we a 50 ohm source to a segment that has an internal
transmission line on it?

Keith A. Lysiak, Ph.D.
Southwest Research Institute

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