[amsat-bb] A word of caution - Hygain DB-218SAT Sat Antenna's
allan at rfdesign.co.za
Sun May 10 22:31:48 PDT 2009
You mention the use of the Hy-Gain Sat Antennas
in you setup. I was foolish enough to purchase this
Oscar Link combination of the VB-216SAT 2m 16 element
cross Yagi and the UB-7030SAT 70cm Cross Yagi.
The antennas themselves are basically OK, but there
are serious technical issues with the "Polarization Switch"
they supply for changing from LHCP to RHCP.
First of all, as supplied by Hy-Gain, these switches
could never work by design.
Some technical explanation - As the T-Match they use on the
Horizontal and Vertical driven element presents an impedance
of 200 ohms this needs to be transformed at the feed point
down to 50 ohms using a 4:1 balanced to unbalanced balun using
a 1/4 wavelength piece of co-axial. These seem cut to the
right length using the thin RG-316.
As per the Satellite Experimenters Handbook and RSGB VHF/UHF handbook, the idea is then to transform this 50 ohms
at the feedpoint up to 100 ohms at the switch using a 1/4 wavelength
run of 75 ohms co-ax (normally RG-6/U). The two 100 ohms from the
V and H will them be in parallel and be 50 ohms again.
So what's wrong with the Hy-Gain switches you ask ?
1: They use RF-303/u 50 Ohm cable from the feedpoint baluns to the
Polarization switch, not 75 Ohm cable. This means that at best
you will get a 2:1 VSWR through the switch, even if the antenna
is perfectly matched. This was confirmed by putting the switch
on a Agilent Network Analyzer which I have at work.
2: The cables they use are of equal length. The problem with this
is that when you open the switch up, the designers neglected to
take into account the extra phase delay through the cheap Tyco
Dc relay they use. This meant the phase match between the H and V
was off by at least 40-90 degrees from the ideal 180. This means
those guys using the switch as it exists will have anything but
To fix the problem I had to practically rebuild the Switching Network by replacing the RG303 with RG-6. The 1/2 wavelength coax
inside the switch used to add 180 degrees phase also has to be replaced with RG-6. The 1/4 wave legs then needed to be adjusted by measuring the phase delay on the Agilent Network Analyzer until you get a perfect 180 degree switch between RHCP and LHCP.
This a way beyond the capabilities of the average Radio Amateur. Emails sent to Hy-Gain (MFJ) about this issue went un-answered off course.
Other issues :
1: I found that the 70cm antenna had a VWSR dip at around
440 MHz and nothing I did with the match could get it any lower
2: On one of the feed point baluns the "Red" marking on the
solder lugs was different from the other 3. This means if assembled
as instructed, you would never get circular polarization. There
is no way I could have know this. It was only discovered when I picked out the Silicone RTV from the balun and saw the
manufacturing mistake on their part.
3: I seriously question the Hy-Gain claim on the data Sheet
"True RF, 50 Ohm switching relays are rated for 200W PEP and contribute to improved VSWR". They use cheap Tyco DC switching
relays in the design and they are definitely not RF Relays.
My intention is to replace the antennas with the M2 versions
in the summer. From the data sheets and correspondence I have
has with them it looks like they have done the job properly.
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