DG7YBN / xpol Yagis
  Last Update Jan 19th 2017




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Circular Polarisation for xpol Yagis


Coax Length Calculator
Phasing xpol Yagis for map65
Leading the Feedlines
Sample Builts




What makes a circular polarised Yagi?


Usually two identical single polarisation antennas are being set up for 90 degr. of polarisation difference.
These two antennas are excited with a 90 degr. of phase shift. With this now a rotating field is radiated.

Depending on the sense of rotation this can either be rotating clockwise (Left Hand Polarisation) or counter clock
wise (Right Hand Polarisation). Terminology is as following: the wave is being radiated from the source,
we follow the wave spreading which for the Yagi-Uda is of relevance in beam direction.

Right Hand Circular Polarisation (RHCP)

RHCP is clockwise in beam direction (red = connected to core of coax)
(1) vertical plane first



(2) horizontal plane first



Left Hand Circular Polarisation (LHCP)

LHCP is counter clockwise in beam direction (red = connected to core of coax)
(1) vertical plane first



(2) horizontal plane first



How to read these drawings


Red arms of dipoles are where to coax core gets attached to.
Grey ones are connected to the shield / braid side.


Looking from the reflector forward ... first dipole sends out a wave, we start at phase 0 degr.
the second dipoles wave meets the first dipoles one when that is a 90 degr. already.

Vertical dipole straight (core on upper side), horizontal dipole (core on right side) thru the 1/4 lambda (90 phase lag) results in RHCP.
Horizontal dipole (core on right side) straight, vertical dipole (core on upper side) thru the 1/4 lambda (90 phase lag) results in LHCP.

At a straight geometrical shift of 1/4 λ = 90 degr. the necessary phase shift is achieved by the
mechanical built alone. The opposite extreme is with both antennas without any geometrical
shift. Here the necessary 90 degr. of phase shift has to be introduced with a full 1/4 λ = 90 degr.
of extra feedline coax length that adds the missing runtime to the second system.

The other dipole must be excited in a way that the first dipoles wave meets the other dipoles wave
when it has 'travelled' a 90 degr. already; this in a picture is like singing a canon, a round.

• Introducing 180 degr. (1/2 λ) instead of the 90 degr. (1/4 λ) makes just a reversal of hands.



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Basics: Unit Circle and Wavelength λ



90 degr is just 1/4 λ. Any multiple of 90 degr. makes a multiple of 1/4 λ.

From 360 degr. on the circle starts again.

Any 360 degr + x degr. is same as just x degrees because of the periodicity of sinus and cosinus.
That is why 450 degr. (if that would exist) makes just 90 degr. or 1/4 λ.








Offset between Polarisation Planes


Most xpol Yagis have an offset between their polarisation planes (on left). If not we call them COPLANAR (on right).
If you only operate either horizontal or vertical polarisation at a time the offset of dipoles on boom and feedline lengths ar no issue at all.

Feeding a Yagi that has a geometrical offset between the planes of 90 degrees with equal phases from equal lengths of coax makes
a fine circular polarisation (left hand here) since the needed shift is 'built in'.
Feeding one dipole in the coplanar arrangement (on right) with a phase lag of 90 degrees by prolonging its feedline by 1/4 λ makes a fine circular polarisation as well.



What if the geometrical built does not meet one of the extremes?


Mind: Two identical single polarisation antennas are being set up for 90 degr. of polarisation difference.
These two antennas are excited with a 90 degr. of phase shift. With this now a rotating field is radiated.

How to figure out the necessary length of phasing coax to achieve real circular polarisation for any geometrical offset?

Calculate the extra length of coax feeding
the other dipole for circular polarisation





Extra Coax length Calc. for xpol Yagis

   Freq. (MHz)
   
        Offset between planes (a)
        mm
      v-factor   or  Use
     
Coax Make & v-factor (1 Mhz)




Lambda (mm)
mm
Phase angle DE to DE by geometry
degr.
Phase angle of extra coax
degr.
Extra length of coax
mm



Notes:

• Negative length for extra coax do occure from a geometrical distance larger 1/4 λ.

Which is correct and means that now instead of extra coax length the supply coax must be shortened
to have the waves of first and other dipole meet at the necessary 90 degr. of phase as defined.

LMR 240 has same v-factor as LMR240UF
LMR 400 has same v-factor as LMR400UF

• Frequency dependancy of plastics: changed velocity factor for VHF / UHF

As explained on the 'Phasing & Matching' website the v-factor of plastics, here the inner coax insulation
is given at 1 MHz. For VHF/UHF it has higher numbers. A rule of thumb is to shorten results based on the 1 MHz
numbers by 7 percent for PE, 5 percent for PTFE coax, less for air foam coax.
A trim on the VNA is better than just cutting by rule of thumb, see here







Phasing xpol Yagis for map65



There are 2 sources I pulled knowledge from. The prolific Rainer Bertelsmaier, DJ9BV and Leif Åsbrink, SM5BSZ have set up schemes
how to phase and feed xpol Yagis in the late 1990ths. I do recommend to read both, especially SM5BSZ's notes on the benefits of X instead
of + configuration of the Yagi, before you set to works.

DJ9BV, Switchable Polarisiation For Cross Yagis, Dubus 1/1996

SM 5 BSZ - Simple Polarisation Switching, link to website
SM 5 BSZ - Separate Polarisation Switching for Rx and Tx, link to website


Most operators will set up the + configuration as they want to use a clear hpol when working tropo with their array.
This is the first kink in the complexe matters of phasing xpol Yagis. Because it is clearly possible to get hpol and vpol polarisation
using the X - configuration. SM5BSZ shows how .

However, the 'modern' approach in majority will be TXing either RHCP or LHCP constantly, so you can be heared worst -3 dB whatever Faraday Rotation
is up or not. And feed the RX lines to either switchable hpol / vpol or dual receiver system. Which might be followed by a map65 dual channel decode
software.

So I show a simple coax wiring for use of a dual receiver setup.
If you do not run a map65 and dual channel sound card etc. you can use two separate radios and PCs running 2 WSJT programs,
or have a manual switch to chose between the RX coaxes leading to a single receiver.

Phasing xpol Yagis for map65

The map65 askes for a phasing angle during setup. This is now 90 degr. in this proposed configuration.


Phasing xpol Yagis for map65 with abilty to transmit RHCP and LHCP

Note: The blue coax length must in total be same length by phase angle as the (2) is.

That does include the coax relays, connectors, everthing. This might be acheived by measuring the ready made ensambe with a VNA.
You will get a resonate spot at some frequency corresponding with the ensemble as if being a 1/4 λ stub. Now dress the (2)
coax to be same.

Find hints for measuring a coax stub resonance here







Leading the Feedlines


How to lead the feeding coax lines for least interference with the Yagi's structure








Sample Builds


GTV 2-8w XPOL by DJ4TC, coax feedlines to come in from the rear


The geomatrical shift between planes is 80 mm or 13.8 degrees of phase on 144.1 MHz. For driving that Yagi with
a Circular Polarisation need to prolong on of the feeding coax lines by 76.2 degrees to make the full 90 degrees.


With a H2010 coax (v-factor at 1 MHz = 0.83) we need to prolong one planes feed line by 365 mm.
Taking onto account that that v-factor growns with frequency and is around 5 percent lower at 144 MHz than on 1 MHz
we end up with a needed length of 348 mm.

But yet again, be adivised to measure the length yourself if you have a VNA at hand:
76.2 degrees of 144 MHz is (144 / 76.2 x 100) = 188.9 MHz to trim to for being resonate as a Quarterwave Stub ...









73, Hartmut, DG7YBN



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