|Reviews Summary for National RF Quadrafilar Helix, weather satellite antenna
Average rating: 5.0/5
Description: Weather satellite antenna, model WX-137Q, for reception of APT satellite images in the 137 Mhz band. Quadrafilar design, omni-directional.
Product is in production.
More info: http://www.nationalrf.com/satellite-tenna.htm
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Mar 20, 2010 16:34
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Excellent Weather Satellite Antenna
Time owned: 0 to 3 months
I just installed my National RF QHA antenna at 15 feet and the result is excellent. I was using a home made QHA made from copper tubing, and I just could not tune it physically to have it resonance on one frequency as some internet sites dealing with building it indicate that the matching network is critical to obtain one resonance frequency. Ordered the National RF QHA hoping the engineering and production would be exact and it was right on the money. Sturdily constructed with non-corrosive materials and I even had special feed line and terminal coaxial connector substituted for no extra price. Delivery time was very reasonable and realistic. Assembly was straight forward with no problems with specific instructions. The National RF Inc owner was also very pleasant and professional. The experience so far has been great.
Mar 9, 2005 07:39
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Time owned: 0 to 3 months
National RF QHA
I think it may be useful for weather satellite enthusiasts to be aware
of a fine and reasonably priced QHA. I am totally in favor of
homebrew, but when it comes to a QHA it takes a fair amount of skill
to get it to work as it should. I don't have the skill or
anymore for that kind of craftwork. More about this later.
I have searched all over for commercial QHA's but always found
to be very expensive. Somehow, I discovered that National RF provides
both a turnstile and QHA among their products. An in-line preamp is
also available. Here is a link to their homepage:
Click on products and find your way to the satellite antennas.
The friendly guy at National told me they do not keep these antennas
in stock and make them to order, because they do not get many calls
for them. No surprise, as this antenna must be the best kept secret
in the weather satellite world. So anyway, it took about 2 ½
take delivery on it from the time I ordered it. It comes packed in a
long narrow box. The individual elements are easily installed by
sliding them into brass tubes at the top and bottom of the main
housing. This keeps the package size down and also allows for
fine-tuning the antenna to resonance if need be. The main body is
made from a high quality gloss PVC and the elements are solid
stainless steel that are nearly impossible to deform unless you put
them in a vice! The elements are held in place by stainless screws
set through the brass tubes. Even though these metals will not
tarnish, I elected to seal the tubes and screws with a dab of RTV ( I
hate going up on the roof to clean contacts! )
The rest of the antenna is already sealed off with RTV and National
claims the QHA is "marine quality"… I believe it!
Connection to the antenna is via a length of, unfortunately, RG58/u
about 3 feet long. I told them at National I would prefer a short
length of RG8X instead and theRG58 and they said they would provide
it, but they did not. In addition I asked for a PL-259 to be
installed on the coax so I could connect my LMR400 line to it via a
double female barrel connector. They did provide the PL-259, but it
was a disaster. No matter, because I cut the pathetic RG58 within 4
inches of the antenna and installed my own Gold Amphenol. But it was
a bit disappointing that such a fine antenna would have these
shortcomings. The PL-259 they provided was all cold soldered and came
apart in my hand with a simple twist of the reducer, which was not
even screwed in all the way. Shame, shame National! And RG8x is a
perfect low loss, low profile, and more durable line to act as a
connecting tail than RG58. Thank goodness it was a "decent"
that the shielding seemed to be near 100%.
Moving on, lets talk about how to mount the thing. I neglected to ask
this when I placed the order and there is no info on this on their web
page. It turns out that at the bottom and center of the main housing
is a female ¾" bushing. So, one would think then that a PVC pipe
a male ¾" adapter would work nicely. Well, only if you like you
antenna to wobble around in the breeze! This QHA is far too top
heavy for that approach. So, I though maybe a ¾" to 1"
be better. The 1" PVC pipe still wobbled, even at a length of
feet. I don't like limp and wobbly anymore than I like RG58! So
is what I did; I took a 3" galvanized steel "L" bracket
and drilled a
¾" hole in the top center. On the side I drilled 2 holes to fit
standard "U" bolt and clamp assembly, the kind you can find
for TV antenna mounting. The QHA is mounted to the bracket with a
closed end ¾" bushing, 2 metal electrical conduit lock washers,
round metal conduit washers to add stability. This gets mounted to
my 6" steel pole that was already up for my previous antenna, via
U bolt assembly. Nice a sturdy, no limpy wobbles!
Now the good part..performance!
My previous antenna was an M2 Eggbeater with the optional radial
groundplane, designed for the 2 meter band. It has been widely used
by Hams who are also weather satellite enthusiasts as it performs very
well and eliminates the need for another VHF antenna. The M2 is a
great antenna, outperforming 2 other turnstiles that I have had up
over the years. I could easily obtain AOS at 4 or 5 degrees with this
antenna. However, I always experienced an annoying dropout at
elevations as high as 18 degrees and other moderate noise lines that
occurred here and there. When I put up the National QHA the
improvement was immediate and enormous. I can now obtain AOS at 1 or
2 degrees, depending on the pass, and have a totally noise free image
until LOS. For the first time in over 15 years in this hobby I can
obtain images of the US West coast with very good signal
quality..not bad considering I am in the New Jersey shore area and
have a 1000 foot rise of terrain to my west only 1 mile away! A 15
degree West maximum elevation pass provides a real nice image, where
in the past it would be just about useless.
I also find that there is a complete absence of any RFI lines in the
image. You know the kind, they look like wavy lines where the signal
is weakest. Can't complain about this either! Oh, by the way, my
receiver is a Hamtronics R139 and I DO NOT use a preamp. My
transmission line is LMR 400 and is about 40 feet long.
National stated to me that this antenna was tuned to resonate at 50
ohms between 136 and 139 Mhz. I checked. It does. I placed an MFJ
antenna analyzer at the feedpoint of the QHA and found the SWR to be a
flat 1:1 across this range. This is important info and here is why.
First of all, ANY transmission line will only maintain its loss
characteristics into a perfectly matched load. Once you start getting
and SWR up around 2:1 or more, your loss figure can go off the wall
and that cherished low loss line you bought is now as good as shoe
string. In fact, the whole antenna system including the transmission
line can now behave like an untuned random wire, real sucky for VHF
reception and allowing everything from line noise, pagers, public
service RF, and the kitchen sink into your receiver and showing up on
Next, the pattern of the antenna is only maintained at its resonant
frequency. That nice symmetrical pattern that you THINK you are
getting can be a mess at 137 Mhz if the antenna is resonating too far
off ( like my M2 Eggbeater?)
So, you also think that nice turnstile or QHA you built is resonant on
137 Mhz? Why? Because you folowed the directions on how to build it?
Did you measure it on an antenna analyzer and founf that the SWR was
not 10:1? Try building a QHA and have it hit the sweet spot of a
perfect 50 ohm match without tuning it! No way, friend!
Here are some pics of the QHA:
When I get some time I can upload some of my own pics. The National
QHA sells for $169.95, is built tough, and performs to spec. For the
price you can't beat it.
E me if you want to talk about it.
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