[NLRS] 2m Transverter success!

[John P. Toscano]

Dan Larson wrote:
> 
> Now I know what my first project of next spring is going to be: Installing a nice set of
> beams on a rotator on my roof! I want to build these over the winter. Currently I am planning
> on building 2m & 70cm cross-beam yaggis that I can use for both horizontal and 
> vertical. I would like the "harness them" up with both horizontal elements and vertical
> elements active at once. I guess that would make either right of left handed circular
> polarization. Is there a penalty or loss doing it that way? 

Hi, Dan.

Yes, unfortunately, there is (always!) a penalty.

If you connect the horizontal and vertical elements with either a 
physical 1/4-wavelength offset; or with the elements lined up with no 
offset but with a 1/4-wavelength phasing cable inserted in one of the 
feedlines before the two antennas are joined at the Tee or power 
divider, you will get either right-hand or left-hand circular polarity. 
  Which of the two you get is dependent on the direction of the offset 
and which element is connected to the center vs. the shield of the coax. 
  (Without the 1/4 wavelength phasing offset, you will get diagonal 
polarity, which is not normally desirable.)  Circular polarity would be 
an ideal arrangement for some of the satellites, and would be 
essentially a 3 dB penalty on either the terrestrial vertical (FM) or 
horizontal (weak-signal) contacts.  Just imagine that half of your 
energy is being radiated in a plane perpendicular to the plane of the 
other station's antenna, and essentially lost to the communication path.

Of course, if you simply run the horizontal and vertical feeds 
separately to the shack, as you point out, you will have twice as much 
invested in feedline and will also lose circular polarity for the 
satellites.

I guess that the "ideal" approach (assuming that you don't mind the cost 
or complexity) is to make provisions for switchable polarity: Left 
Circular, Right Circular, Horizontal, and Vertical.  Probably the best 
way to do this is by placing the antenna elements in an X configuration 
instead of a + configuration, and with 4 coaxial relays, 4 phasing lines 
(two of them 1/4 wavelength long, one of them 1/2 wavelength long, and 
one of them 3/4 wavelength long), you can obtain all four mentioned 
polarities.  Oh yeah, you will also need a 2-port power divider to keep 
the impedance at 50 ohms, since you will always have two 50 ohm antennas 
"in parallel" to one another, giving only 25 ohms and a poor match.  If 
you need to know more about that, say the word and I will be happy to 
explain how it is wired up.

Matt Stahl at M-Squared talked me out of that approach for my station 
after a fairly long telephone conversation with me a couple of years 
ago.  What he and I designed for my home setup (although I bought all 
the pieces but haven't put them onto the tower yet!!!!) is as follows:

   1) Instead of an antenna with circular polarity, stack two identical
      antennas, oriented horizontally, one above the other.  This gives
      3 dB of gain over a single antenna, which is essentially as good
      on average as having circular polarity when working the satellites,
      and is twice as good for working weak-signal stations.  No relays,
      no switching, just the antennas, a two-port power divider, and
      two identical phasing lines.  (The phasing lines could be any
      length, in theory, so long as they are identical in length, but in
      practice it is usual to make them an odd number of half-wavelengths
      long, i.e. 1/2 or 3/2.)

   2) One pair of stacked 2M12's covers the 2 meter band.  One pair of
      stacked 432-9WL's covers the 70cm band.  The mast coming out of the
      tower will have an elevation rotor at its top, with a cross-boom (I
      bought the 10-foot M-Squared fiberglass crossboom) horizontally,
      and on each end of the crossboom is a 12-foot aluminum mast (picked
      up at Garelick Steel in downtown Minneapolis) attached vertically.
      The two 2M12's are mounted at its ends, i.e. 12 feet apart.  The
      two 432-9WL's are mounted 6 feet apart, leaving another 3 feet of
      vertical mast going up and down.  On those two ends will go loop
      yagis for 1269 MHz (L-band uplink to Oscar 40) and 2400 MHz (S-band
      downlink from Oscar 40).  The entire H-frame assembly can be
      rotated in the elevation plane as needed to track the satellites,
      and positioned at no elevation to work terrestrial stations.

   3) The thing that's "missing" from this setup is a beam for FM work.
      I decided that while it might occasionally be handy to have a
      directional antenna for FM work, it is inherently not a DX mode of
      operation, and my Cushcraft ARX-270N actually performs very nicely
      for my FM needs.  (My current "temporary" setup, not the eventual
      setup described above, includes a 5+5 element dual-band yagi from
      Cushcraft that is mounted vertically.  It seldom works any better
      than the ARX-270N, and so I don't plan to keep an FM beam in the
      final configuration.)

Besides the loss of a directional FM antenna, I admit that there is a 
small degree of compromise on the satellite functionality.  Even 
satellites with circular polarity antennas on them do not act like they 
have pure circular polarity for the entire pass, due to squint angles 
and the like.  Also, if the satellite is rotating, the non-circularity 
becomes noticeable as cyclic fading to a station with a linear polarity 
antenna, and circular polarity on your home system will help with that 
to some degree.  So if you're looking for the ultimate satellite antenna 
array, by all means go with the switchable polarity setup, and maybe 
even with a stacked pair of circular polarity antennas per band!

73 de W�JT