[NLRS] 2m Transverter success!

[Dan Larson]

On Mon, 29 Sep 2003 20:29:35 -0500, John P. Toscano wrote:

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

Dang! I thought there was a free lunch here somewhere <G>


>
>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.

OK. I forgot about the 3db between circular and either horizontal &
vertical.

>
>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.
>

Maybe I should be think about separate arrays now for terrestrial &
non-terrestrial.

>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!
>

This is what I had originally envisioned, but I wanted to make the
whole aparatus
smaller. Its either a big H frame with multiple beams per band or a
huge expensive 
box of coax relays! (I told you I wanted a free lunch! <G>)

Dan KC0LUY