TV yagis for UHF radio astronomy work

[linear_shift]

Hi all,

I am currently constructing a 2.7 by 1.2 meter parabolic cylinder (and
four UHF double bowtie antennas for the feed, likely going to be
changed soon to increase available aperture) for UHF (608-614 MHz
radio astronomy, as many of you know. I have, however come across a
cool yagi, that, with two, could nearly equal the beamshape, and
likely out do the gain of my cylinder:

http://www.tvaerials.com/product.aspx?productid=32

For months before deciding to construct the cylinder, I had considered
TV yagis, but they were lower gain than even by phasing four of them
together, out do my cylinder (it would have been a hell of alot easier
to phase some premade yagis together than build my chicken wire
monstrosity, believe me ;-) ). This opens up new avenues for me like
a portable radio telescope, a motorized alt-az mount, etc, that I
could not do with the cylinder.

However, I would like to ask SARA, does that antenna have potential?
That is to say are its advertised gain figures, front to back ratio,
beamwidth, etc. within reason of this antenna? The gain of the yagi
was advertised at 19 dBi, when a dish (at 100% efficiency, I'm
assuming yagis don't have big aperture efficiency concerns) of the
same aperture gets around 17 dBi gain. I am aware that yagis have an
elliptical, rather than circular aperture, but given that the long
axis, the dimension of which I got from wavelength /
beamwidth_in_radians (0.491 meters / 0.383 radians) is what I
calculated the equivalent dish diameter dish from, and that the other
aperture axis (in the H plane) is always smaller for a horizontally
polarized antenna, the gain should actually be slightly less than
this. Maybe the advertised gain figure is for the top of the band
(can't really tell, as the aperture may be different for the other
frequencies in its bandwidth)?

Anyway, this brings me back to you guys. What do you think of this
antenna? My cylinder gets around 19-20 dBi gain, so if two of those
can beat this mass of chicken wire, they're bought (eventually, still
trying to dredge up funds to complete the cylinder for meridian
transit operation) ;-D

As always, my website is:

http://channel37.110mb.com/

And my blog is:

http://kmradioastro.blogspot.com/

Anyway, thanks in advance all your replies,

KM.

P.S.

I haven't tried actually building a narrowband long yagi for 608-614
MHz, because I intend to utilize some potentially quiet space in the
former part of the UHF TV band (the analog UHF space between 700 and
860 MHz), which the blaring megawatt TV transmitters will be dropping
from in June (Arecibo is building an 800 MHz receiver on their dish
for after this event as well). Thats basically why I want to have a
wideband antenna, well, that and I want to try some EME TV/DX
reception :-P

[KD7JYK DM09]

"608-614 MHz"

Isn't that the medical telemetry band?

Kurt

[Marko Cebokli]

Hello,

I am afraid these are "Marketing" decibels....
19dBi you get with a narrowband yagi cca 8..9 wavelengths long!

http://www.ifwtech.co.uk/g3sek/diy-yagi/index.htm

These "Double V" shaped elements give you some more bandwidth, but not much
more gain. This type of TV antennas was usually sold in two versions, for the
upper and lower half of the TV UHF band, and even these had cca 3..4dB of
gain variation within their sub-bands, with max gain close to the upper band
edge. So one for the full UHF band must stink pretty well at the lower end!

Marko Cebokli

[Jan Lustrup LA3EQ]

I agree with Marko....I have owned a few of these antennas here in Norway
many years ago to watch TV on UHF when there was "tropo" og sea ducting from
the UK under high pressure in the North Sea.(no Norwegain UHF television at
that time)........The high gain claims are for the salesmen only...they are
not true accross the band.....best at the top of the band..The balun is
lossy too(high return loss). And high winds love to tear them apart, not to
mention birds love to sitt on the elements and deform them (at least the big
fat birds)..But they might last longer in the dry weather of Arizona...
Jan Lustrup LA3EQ
Norway

[linear_shift]

Ok, thats what I was dreading/hoping. On one hand I was hoping to use
these suckers for a more compact UHF radio telescope, and on the other
I was hoping that my cylinder wasn't going to be outgunned ;-D Its
settled then, the cylinder is victorious. :-P

Kurt,

608-614 MHz is a shared band for radio astronomy and low power (I
guess you could call "QRP") medical telemetry (1 watt ERP or less). I
chose it because of the nature of my observations, availability of
cheap tuners, easy to construct LNAs (not the commercial TV preamps,
but extremely low noise ones you can make out of GaAsFETs intended for
satellite Ku and C band reception), and ready made antennas for dish
and cylinder feeds.

As for the cylinder, I intend to make a new feed soon, basically a
collinear array of 7 bowtie antennas

http://www.radioshack.com/product/index.jsp?productId=2062017

mounted on a piece of PVC pipe, with four phased two in series two in
parallel, and the other three phased two in series two in parallel
with the combined four,

http://www.hdtvprimer.com/ANTENNAS/ganging.html

to replace the large blockage and mediocre SWR of the double bows. It
won't have any sort of reflector on it at first, but I might put a rod
reflector on it to reduce spillover later. This new setup should
greatly enhance gain, maybe hitting the intended 65% or better
efficiency typical of cylinders (since you only need to worry about H-
plane illumination and having a full line feed, rather than the
messing around with taper in both planes for a dish). Right now it is
probably at around the equivalent of 50% due to the blockage of the
reflector screen, and some diffraction effects (noticed a few,
relatively strong, maybe -10 dB sidelobes in the H-plane when testing
it with Jan's noise source, which seems to indicate diffraction). That
being said, the cylinder seems to be working quite well, despite the
large feed, and should be a spectacular antenna after the feed
switch :-) :-)

Anyway, I'll be updating my blog informing the public I'm not dumping
the cylinder :-P

Thanks once again,

--Kurtis

[linear_shift]

Correction, I'm going to use this one,

http://www.tedss.com/14-7480/

for the feed seen as how RadioShat doesn't appear to ever have them in
stock and are overpriced anyway :-P

[Jan Lustrup LA3EQ]

HI,
I took a look at the bowties at http://www.tedss.com/14-7480/.
These are 300 Ohm impedance units...So watch out when making a 300
Ohm-75Ohm, phasing harness....it's easy to make a mistake.
Why not keep to your orginate feeds/phasing harness and just remove the feed
reflector screeens?
It would save you a lot of work and some $$$.
A tip: make a loop of 3 to 4 turns (5 inch diameter)on you coax close the
the antenna to stop RF noise from your computer entering the antenna via the
coax screen or else you might be seeing broadband PC hash noise drowing out
the weak space signals.....try and move the antenna around a bit to see if
you can detect any changes in background noise, and choose the "sweet spot"
with minimum noise pick up.
Looking forward for your drift scan results....Sun, Cassiopeia.A and Taurus
A signals....etc
Good luck...

[linear_shift]

Jan,

One of the main reasons I want to use the new feed, is its extended
line coverage. The current feed should only get about a 12 degree
horizontal beamwidth. With a _full_ line feed I could get 10 degrees,
and a better efficiency. When I tested the antenna I got a 9 degree
horizontal beamwidth, but honestly, I don't know how that is possible
(it was a TV reception test, and I had not yet received your noise
source). The feed gain would indicate a full line feed in terms of its
gain and H-plane beamwidth (14 dBi and a 75 degree H-plane HPBW, comes
out to have a 2.7 meter long aperture and thus 10 degree beamwidth),
but the physical aperture of the feed is only two meters long (it'd be
nice if it is stretching out to the end of the cylinder, for some free
aperture coverage), so I don't know how that is possible (yagi-like
ray bending maybe?).

Some of the other reasons are, weight (the current feed adds about 15
kg to the antenna itself, and with the mount design I'm using the
weight is a concern), blockage of course, and combiner loss (the
current feed uses a four way 75 ohm splitter, with the 300 ohm phasing
harness, less balun, there will be no loss, if executed properly).
Another reason are the sidelobes I got (seemed like around -10 dB,
again no quantitative tests since the TV I had with the video output
broke, and I still can't afford a new soldering iron to put my
receiver together with, or the parts for the power supply), which are
likely caused by knife edge diffraction from the feed screens,
alhough, possibly just from the diminutive size of the cylinder
compared to wavelength, the close focus of the feed, or a combonation
of all three. As far as just taking off the reflector goes, you
actually still have the blockage there, in the antenna aperture. Most
of received energy would be from the the feed itself, rather than the
reflector. Another pertainant reason is the beam shaping system, where
with the wider beamwidth of each element allows more steering room
(the beamwidth of the double bows is about 50 degrees in the E plane,
as opposed to 72 degrees for a single dipole without screen).

Anyway, there is still a lot of time for me to decide on what to do
here with the feed, since some usable funds (not going to the family)
are acquired here (I'm selling some stuff to do just that, anyone
wanna buy a Fender Nashville Telecaster? :-D).

Thanks once again,

KM.

On May 5, 1:42 am, Jan Lustrup LA3EQ <lust...@start.no> wrote:
> HI,

> I took a look at the bowties athttp://www.tedss.com/14-7480/.

[linear_shift]

Oops, sorry was looking at the wrong band on the polar plot. Its about
100 degrees in the H-plane for the feed at my freqs, and the gain of
the double bow is 9 dBi, so 15 dBi gain for all four without combiner
loss. Was wondering how the hell I could have gotten 2.7 meters from
that for the line feed length, good thing I keep the old pieces of
paper I scribble my calculations on :-D

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[Jan Lustrup LA3EQ]

Ok Kurt....
And remenber,,,,,don't calulate (or add) in the "gain" of the feed.....all
the "gain" comes only from the reflector capture area ;)
Good luck.....
Jan

[linear_shift]

Jan,

Oh yeah, I know about that. I was just using the gain of the feed by
itself and the beamwidth to determine the aperture dimensions and
shape, so as to find out the focus line coverage (how full the line
feed is). As I stated previously, I do not know how the hell the
double bows are getting aperture outside the screens. The new feed
should solve alot of problems though. As for money, I only need about
$20.00 for those bowties, a 4:1 300 to 75 ohm balun, and a piece of
PVC or fiberglass. In retrospect it would have been alot cheaper to do
this in the first place (the current feed cost about $45 overall plus
shipping) :-P I should be able to to pull of the 300 ohm harness, as
the twinleads one the bowties are just about the right length to not
be overly long and shouldn't be able to touch plastics and metals in
the cylinder (I might buy some more twinlead from somewhere, in case
the ones on the bowties aren't long enough).

Cheers,

KM.

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