A Modifed Antenna Craft Y10-7-13

The primary challenge at the receive location was to improve response for VHF Channel 13, which is/was the weakest of 7, 8, and 13 serving the area.

Initially a 5 director extension was implemented (referenced as ModII), but never put into service because of local high wind conditions.  For some months, however, a version implementing a 3 director extension (ModIII)was used.

Here's a couple pix of the ModIII version in service:
 

 


All of the images on this page will zoom in/out with the scroll wheel up/down (on Windows, dunno 'bout Mac).

The modifications consist of:
a) A planar reflector, and
b) A 5 director extension.

This is a top view of the 5 director extension (ModII version),


and here is a rear view.


The test transmit antenna can be seen in the distance.  Its about 50' away.

Snow Effects
What happens when you load an antenna with snow?  Betwen the previous day, and to the early morning when this picture was taken, and to later the same afternoon when almost all the snow had fallen off, the reception "quality" indicators on 4 different model DTV's varied less than a few points, for either antenna.

Gain Improvement
The general increase in sensitivity ranges from about 2.5 to 4 dB across the VHF Hi-Band,
with the most significant improvements at the extreme low and high ends.

Before presenting response captures, however, a few notes on the measurement process are in order.

• None of the  below captures are intended to present the actual (absolute) field response of the Y10-7-13.
• Each result is presented as a comparison between two antennas, in a test bed with otherwise identical electrical and physical aspects.  That is, the same Tx and Rx locations were used, the same cables, baluns, Tx levels, etc.  The single variable was the unit under test (the stock and modified antenna).
• The antenna test bed is by no means ideal.  Almost every placement of Tx and Rx antenna at these close distances and proximity to ground can result in reflections that affect absolute receive levels at various points across the spectrum, and to varying degrees.  The general rule of thumb, however, is that Tx and Rx antennas should be at least 10 wavelengths apart, and that rule is honored.
• The test bed was established by evaluating results in both horizontal and vertical polarizations.  I.e., after characterizing a response with, say, a horizontal polarization, both Tx and Rx antennas were rotated 90 degrees to a vertical polarization.  If the results varied significantly the Tx and/or Rx locations were changed.  Both horizontal and vertical measurements were repeated, and locations adjusted again if necessary, until locations that produce similar results in both orientations were found.  It is an accepted practice in antenna testing that if the results are similar for both orientations one can be reasonably assured that measured differences are due to unit differences versus environmental differences.

Here is an overall screen capture comparing the modified to stock response:


Here is another capture, form a different location, comparing overall gain with a capture of the local Ch 7, 8, and 13 signals merged in.


The following captures provide closer views of the individual channel responses.  Note, however, that they are taken at yet another location, where the OTA signals were adequate.

Channesl 7, 8


Channel 13


This last capture was the fruit of the project.  The gain improvement is obvious, but noteworthy
is tha the "quality" improvement was about 5 to 8 points, whatever a "point" is (there seems to
be no standard for how various manufactures aggregate total receive signal strength, flatness,
error rate, etc., on their "Signal Quality" sub menus).

Construction
Here are the mechanicals.



  

In addition a support wire was strung from about the rear to about the 3rd additional director.

Summary
I never put this version into service because the location sufffers from occasional 50++ MPH wind gusts.  I did however, put a 3 extension version into service for some time, only to be replaced by a new arrangement 85' up
in a nearby White Pine tree.

The experiment revealed two primary things.  First, signal strength ain't everything... at all!  A much more
significant increase in "quality" was hoped for.  But the fundamental problem is/was not signal strength, but
rather the signal "flatness".  There is another page coming to this website soon titled "Quality versus Quantity"
which discusses the occasional irrelvance of the absolute received power.

The results also suggest the Y10-7-13 can be improved upon.  In particular the stock model's response appeared
to drop off just a little bit early at both low and high extremes of the band, affecting the response for 7 & 13.  Perhaps more modest modifications could correct that.  Mainly, just a slight increase in length of the existing reflector might help the low end, and addition of just one or two directors cut (shorter than the exising last director) will help at the high end; alternatively just shortening the existing last director might help.

While no subsitute for a multipath free signal, for a really deep fringe area situation such modifications might help to
bring signal strength up enough to successfully improve viewing reliability.  Although not equipped to precisely measure actual field strength and gain in absolute terms, a 2.5 to 4 dB gain increase on the published Y10-7-13 specification would put its gain in the same ball park as the Funke PSP-1922, imported from the Netherlands, and which had a published average gain of 14 dB (not 14 dBi).  [But a lot of pages referencing their website seem to have disappeared in the last few months, and googling for "buy Funke PSP-1922 VHF antenna doesn't seem to prodce any vendors.]