VHF R-S VU190XR LPDA (No UHF)

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VHF R-S VU190XR LPDA (No UHF)

VHF (Ch2-13) Radio Shack VU190XR Log Periodic Dipole Array (LPDA) "VEE" Antenna analyzed using 4nec2.

Ken Nist's EZNEC file converted to 4nec2:
http://www.hdtvprimer.com/antennas/comparing.html
Note that Ken Nist's antenna model (see "SIMS" link at top of webpage) is ONLY for the VHF Band
and is MISSING the UHF Elements and associated Corner Reflector.

FIVE versions were analyzed:

a) VU-190XR per Ken Nist's Antenna Model. Note that this has a GAIN HOLE on 213 MHz (Ch13).
Suspecting that the missing Corner Reflector might have an effect, I constructed
an alternative model (not shown), copying (JUST) the Corner Reflector from his VU-75 model
and shifting position on the X-Axis to the same distance in front of the forward element
as is found in his VU-75 model...a sensitivity analysis revealed that moving the
X-Axis location back and forth several inches only caused a few tenths of a dB Gain
difference, and made very little difference whether it was included in the model or NOT.

b) Item a), with Shorting Stub added across Rear Elements (perhaps it's already in the Boom???).
This ELIMINATED the GAIN HOLE on 213 MHz, restoring the expected smooth, decreasing Gain Curve.

c) Item a), but with FOUR MISSING ELEMENTS to assess impact of severe antenna damage.
This exhibited SEVERE GAIN LOSS across entire VHF Band, esp. Ch11-13.

d) Item c), FOUR MISSING ELEMENTS, but with Shorting Stub added. This also exhibited SEVERE
Gain Loss across entire VHF BAND, although not quite as severe as without Shorting Stub.

e) FOUR MISSING ELEMENTS with remaining Elements shortened for the "best" Raw Gain as
determined using nikiml's Python Optimization Scripts.
Remaining Element PAIRS were shortened the SAME amount & NO Shorting Stub needed.

As was addressed in Cebik's "VEE" LPDA Analysis paper, in a Ch2-13 Antenna the elements
resonate at Lamda/2 for Lo-Band VHF Channels and 3*Lamda/2 for Hi-Band VHF Channels,
which results in significantly higher Raw Gain for the Hi-Band than a smaller Lamda/2 sized
antenna [another example of the "Bigger is Better" rule.....]:
http://w4rnl.net46.net/download/v1.pdf

So it was no surprise that SOME of the Elements were close to their original full length....and
others were 1/4 to 1/2 length. However, the Optimization also resulted in the two most forward
Elements being at the MINIMUM allowable length (I stipulated 1-in to avoid segmentation problems
in the optimization process)...so those particular X-Axis placements must be causing a problem,
rather than contributing to the overall Gain of the Antenna:

Optimized performance was much better after the Elements were cut, and although Azimuth Pattern
was irregular, overall performance was quite Good:

Hi-VHF Raw Gain = 9.6 dBi +/- 0.3 dB, F/B & F/R Ratio Min = 16.2 dB (GOOD) and SWR Under 1.3.

Date(s): 25 Aug 2013. Album by holl_ands. 1 - 50 of 50 Total. 2576 Visits.

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