A coaxial collinear antenna for 70 CM

this coaxial collinear antenna was built to test for my own consumption whether or not this antenna method actually works.

the guiding principle is that we have multiple 1/4 wave elements fed in series , the radiation from each adds in phase to produce
a large vertically polarised omni-directional antenna with substantial gain.

My problem in thinking about this antenna is that the simple theory is all well and good in principle.  The practice is
more concerning, especially when you consider the phasing relationships.

The problem lies in the differant wavelengths of the free space outer element compared with the fact that the inner coaxial element
( it is still a transmission line !) the wavelength can be 60 to 80 percent differant. what happens to the phasing model as seen from
a distant place, like the station you are trying to talk to ?

The literature says that to achieve the correct phasing you cut the 1/4 wave radiating elements for the phase shift as seen by the the transmission line.

 This makes the radiating elements (outer coaxial sheaths) the wrong length !  That is a non resonant length presenting a high SWR to each coaxial element...all in series !  Furthermore, the phasing as seen by a remote observer becomes skewed.
SWR matters at UHF, it is turning precious watts into British Thermal Units, not radiation !

This antenna was an attempt to seperate hype from reality.  My conclusion is that the hype does not live up to the reality !
The gain measured in practice , at 70cms, was less then just using my 2 meter (146Mhz) J Pole antenna on 70,  which is just plain wrong!

So what was wrong with my collinear ?  There are TEN radiating elements here, including the stick at the top and the brass sleeve decoupling
assembly below.  The sleeve was cut to resonate with the outer braid, accounting for the dilectric constant of the PVC outer insulation.
This is the low loss method. I have seen in the literature the use of coaxial ferrite sleeves to eliminate coaxial outer currents, but please,
these sleeves eliminate SWR but at the cost of turning to useless heat all those carefully generated RF watts and precious recieved picowatts.
Thats why I used a classical 1/4 wave decoupling sleeve.  A better decoupling method is to use a quarter wave groundplane or 1/4 wavelength radius
circle (not a big ask at 70cms) however I could not solve the mechanical problem of mounting it to its pvc pipe radome.

Humpth! I will not make a coaxial collinear again.

However, supposing I make my own AIR SPACED coaxial assemblies. The inner and outer phasing relationships would be always correct
and maybe the hype and reality might line up for once.  I am debating with myself if I use copper or aluminium coaxial elements. I dont know which would be easier to produce. I can easily braze aluminum but brazing copper needs something better than a propane torch.

Watch this space !

The coaxial cable was quality low loss,1/2inch  foam di-electric Belden 50ohm Ethernet Cable. A foam dilectric cable was chosen as its internal wavelength would be a closer match to the free space wavelength.

coaxial array and decoupling sleevestick at the top and decoupling sleevedecoupling sleeve
method of attaching decouling sleeveMeter Stick ruler for comparisonmounted into its pvc pipe radome


page created Mon Mar  5 18:22:23 EST 2012