blew my second WBF front end after 1 day of wonderfull RX. I don’t know
how it happend. I was using a few different types vhf and uhf omnis and
yagi antennas, everything seemed to be working fine. Now it can’t hear
hardly anything and the typical strong noise signal in the center freq
is back. Is it possible that some of the equipment mounted on the
antenna mast could be shorting it out? Or is the WBX just so sensitive
that a strong signal can blow it up? I have 2 Ubiquiti bulllet2HP’s on
the roof. One is clamped to the same mast as the UHF omni that was
connected the to WBX. They are 24V DC over the unused pairs. Im really
bummed out. Any recommendations or similar experiences ?
On 04/12/2012 08:11 PM, dave k wrote:
blew my second WBF front end after 1 day of wonderfull RX. I don’t know how it
happend. I was using a few different types vhf and uhf omnis and yagi antennas,
everything seemed to be working fine. Now it can’t hear hardly anything and the
typical strong noise signal in the center freq is back. Is it possible that some
of the equipment mounted on the antenna mast could be shorting it out? Or is the
WBX just so sensitive that a strong signal can blow it up? I have 2 Ubiquiti
bulllet2HP’s on the roof. One is clamped to the same mast as the UHF omni that was
connected the to WBX. They are 24V DC over the unused pairs. Im really bummed out.
Any recommendations or similar experiences ?
Unfortunately, LNAs are notoriously sensitive little princesses. The
new revs of the WBX have TVS diodes between the RF connector and the
LNA,
which should reduce this problem, at the expense of a slightly poorer
noise figure. If the local field strength of RF on your roof is high,
enough
can couple in to damage the LNA. In those situations a limiter might
be needed, or even a stiff bandpass filter to keep stray RF fields that
are out-of-band with respect to your application out of the
front-end.
There’s a company online, summitsource, that sells in-line gas-discharge
surge arrestors for CATV/SATV use that I’ve used for protecting
LNAs in the past. Taken local lightning hits and survived, for what
it’s worth. The arrestors are cheap–under $4.00 apiece as I recall,
and
have 75-ohm connectors. About 0.3dB insertion loss.
–
Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
thanks Marcus. Some of the signals i am monitoring are very
strong-@860mhz approx 1/2 mile from my receive antenna. I would say from
memory they were -30 or maybe -20dbm? Is this to much? My gut tells me
no. Is it possible to cause damage by simply turning up the gain to high
on a strong signal?
I am going to remove all the WiFi stuff from the mast with my antennas.
Is all i have to do is solder a new LNA to the WBXFE daughtcard? part
numbers?
Im with you on the sure arrestors. I do have a polyphasers impulse
suppressors in line with all the feed lines coming in for lightning
protection but that’s a whole different issues.
A nearby lightning strike (but far from a direct hit) took out the LNA
on my WBX last fall. I suspect that part of the reason was that the
antenna I was using (a discone wide-band antenna mounted on the roof) is
not DC grounded and thus can develop a good sized charge.
An anecdote: many years ago I built a ham radio repeater system that was
located on the roof of a dormitory building at school. There was a
short tower on the roof and the 1/4 wave ground plane antenna was
probably about 120 feet above ground. The coax cable went from the
antennas down to our ham shack on the top floor of the building. At the
bottom of the surplus CATV hardline cable was a PL-259 connector with a
“barrel” connector to allow hooking up a flexible jumper. The
dielectric between center and shield in the barrel was a clear plastic.
I was working on the repeater one spring day with the antenna
disconnected. Every minute or so I heard a faint “pop” sound. When I
investigated, I saw blue flashes in the plastic on the barrel connector.
It turned out there was storm activity in the vicinity, and the charge
on the 19 inch long whip built up to a level where it arced across the
3/8 inch or so of dielectric. The storm was far enough away that we
couldn’t hear the thunder inside the building.
Since then, I’ve preferred to use DC-grounded antennas (or at least to
have something in the system that provides a DC path)! Sadly, I didn’t
follow my own advice last fall…