Re: [amsat-bb] RE:Livetelemetry 25.6

["Edward R. Cole" <al7eb@xxxxxxxxxxxxx>]

>From: Jerry Pixton <jpixton@shentel.net>
>I also happened to be listening and trying to decode tm with Ao40rcv. Using 
>Phillips Tech dish, (linear but with 44 inch pigtail) with UEK3000. Heard 
>the definite drop out at 23:23.
>
>Can someone please educate me on Helix antennas
>
>  Early on, this list talked about finding aluminum towel rods in the 
>hardware stores and using them for helix supports. What is the effect on 
>metal in the core of the helix?  In my case, the Phillip Tech dish uses a 1 
>inch square tube as the strut for the linear feed. So I was considering 
>getting a longer piece of tubing so I can put the UEK3000 behind the helix 
>with can reflector.
>
>But the tubing is so big - 1 inch that it will almost touch the helix if it 
>has diameter of 1.6 inches. Is this a bad idea? Should I be looking for a 
>poly rod?  There seem to be enough mechanical issues to deal with that I 
>don't want to start down the wrong path using aluminum 1 inch tubing.
>
>
>My plan, right now, is to leave the linear feed in place until after this 
>current orbit adjust and try to get some actual contacts as a baseline. But 
>be ready to put in a helix if things are not good.

Hi Jerry,

The helix I am using has a 5/16 inch bolt running up through the helix axis
and supports a 1 inch I.D. PVC tube to provide a non-conducting support for
the helix.  The bolt runs through a hole in the 3x3 inch aluminum plate
reflector and I have a 1-inch diam. flat washer and nut at the other end to
clamp the PVC tube to the plate.  

Helix literature states that a metal support tube should not affect
performance of the helix as long as it is perfectly centered along the axis
and does not electrically short the helix turns.  So I would provide some
insulator for your metal support.

I modeled my 5-turn helix using NEC4WIN95 without the metal bolt and
optimized it for pattern and impedance match.  Then I added a metal tube to
the model and saw absolutely no change in pattern or impedance.  

I was hoping that his would work out as this is the basis for my design of
a dual-band feed for S/X-band.  The X-band feed is a 3/4 inch diam.
circular feedhorn concentric {it replaces the bolt} to the an s-band helix.
 I will start building the prototype this weekend and play with finding a
mutual phase center {the real trick}.  If successful it will be presented
to MW Update and at Amsat Symposium.

Now all this theoretical talk is fine, but as readers of the list know I
have had trouble hearing the satellite.  I believe most of this has been
due to high NF and high squint angles, but it could also indicate that my
helix is not up to par.  I currently have no equipment for making impedance
or return loss measurements.  My attempts at measuring the beam width are
extremely crude since I merely used the S-meter and the Yaesu G-5400 rotor
readouts.  Later I hope to use a precise compass rose and attenuators to
get better pattern measurements.  I have a directional coupler on-order
that should provide the return loss measurements. 

BTW a dipole will only degrade your antenna gain by 3 dB.  I should work
sufficiently well for experimenting and gaining experience.  I may make one
my self for comparison with my helix feed.

I believe many have discovered that the journey to Mode-S includes fun
building and experimenting with different things.

Ed  
Details on the 5-turn helix design:
The helix turns are approx. 1-1/4 inch diam. and constructed of No. 6 solid
bare copper wire {1/8 inch diam}.  The helix length is 4.09 inches and the
first quarter-turn is a 1/2 by 0.9 inch brass stripline spaced 0.13 inches
from the reflector surface at the N-connector pin and 0.19 inches at the
beginning of the helix wire.  NEC4WIN95 indicates Z = 95.54-j32.99 which
gives a SWR = 1.58 with 75 ohm coax.  Pattern modeling shows a HPBW ~58
deg. and -10 dB bw of ~93 deg.  with a gain of 11.07 dBi.  But as I stated
above, this is the model...real antennas ARE different, so adjustments of
stripline and turn spacing will be required to obtain the desired performance.

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