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HOW do "dual-band"VHF/UHF antennas work?



 
 
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  #1  
Old November 13th 06, 12:11 AM posted to rec.radio.amateur.antenna
Paladin
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Posts: 7
Default HOW do "dual-band"VHF/UHF antennas work?

Hello "Antenna-Lovers",
How do dual-band antennas work? I bought a COMET
"SSB-7" antenna for my mobile radio. It worked great for a year or more
and then,trouble. I don't know "HOW" to trouble-shoot it.
Where on the antenna can I check for "continuity" or somewhere I can
look to find a "fault" of sorts.
It costs a bit more than a lower gain antenna. How can I trouble-shoot
it ?

73's Paladin

  #2  
Old November 13th 06, 04:04 AM posted to rec.radio.amateur.antenna
external usenet poster
 
Posts: 137
Default HOW do "dual-band"VHF/UHF antennas work?

Dual-band mobile antennas in general have some phasing network that
allows the antenna to have a good radiation pattern (gain at the
horizon) on both bands.

They also need to have impedance matching on both bands.

Unfortunately, there's nothing universal about dual-band designs as far
as continuity or shorts or anything like that. Additionally, a search
on "Comet SSB-7" brings up some fairly unfavorable eHam reviews, many
of which mention DOA antennas, having to get replacements twice, etc.

There's not much you can do without knowing all the specifics about the
design, even then it would be tough to fix whatever's wrong; something
you can do is to check are the coax continuity to the NMO mount you're
using.

Dan

  #3  
Old November 13th 06, 05:13 AM posted to rec.radio.amateur.antenna
Jimmie D
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Posts: 296
Default HOW do "dual-band"VHF/UHF antennas work?


wrote in message
oups.com...
Dual-band mobile antennas in general have some phasing network that
allows the antenna to have a good radiation pattern (gain at the
horizon) on both bands.

They also need to have impedance matching on both bands.

Unfortunately, there's nothing universal about dual-band designs as far
as continuity or shorts or anything like that. Additionally, a search
on "Comet SSB-7" brings up some fairly unfavorable eHam reviews, many
of which mention DOA antennas, having to get replacements twice, etc.

There's not much you can do without knowing all the specifics about the
design, even then it would be tough to fix whatever's wrong; something
you can do is to check are the coax continuity to the NMO mount you're
using.

Dan

My Comet antenna went bad too, for grins and giggles I hooked my 2M radio up
to my CB antenna . SWR was good I hit all the local repeaters, Have tried it
on UHF and it works there too. Probably far from optimum performance but I
cant tell a lot of difference between it and the 2m half wave I have used. 1
antenna 3 bands. It works for me. What I think is going on is the bottom
hald of the center loaded CB antenna is about the right length for a 1/4 wl
2m antenna with the loading coil acting pretty much like a choke on 2m and
432. On UHF it probably looks lie a 3/4 wl antenna. Losses in the cable
probably help the match to the radio. What the heck , it works.


  #4  
Old November 13th 06, 03:53 PM posted to rec.radio.amateur.antenna
Mark
external usenet poster
 
Posts: 27
Default HOW do "dual-band"VHF/UHF antennas work?


Jimmie D wrote:
wrote in message
oups.com...
Dual-band mobile antennas in general have some phasing network that
allows the antenna to have a good radiation pattern (gain at the
horizon) on both bands.

They also need to have impedance matching on both bands.

Unfortunately, there's nothing universal about dual-band designs as far
as continuity or shorts or anything like that. Additionally, a search
on "Comet SSB-7" brings up some fairly unfavorable eHam reviews, many
of which mention DOA antennas, having to get replacements twice, etc.

There's not much you can do without knowing all the specifics about the
design, even then it would be tough to fix whatever's wrong; something
you can do is to check are the coax continuity to the NMO mount you're
using.

Dan

My Comet antenna went bad too, for grins and giggles I hooked my 2M radio up
to my CB antenna . SWR was good I hit all the local repeaters, Have tried it
on UHF and it works there too. Probably far from optimum performance but I
cant tell a lot of difference between it and the 2m half wave I have used. 1
antenna 3 bands. It works for me. What I think is going on is the bottom
hald of the center loaded CB antenna is about the right length for a 1/4 wl
2m antenna with the loading coil acting pretty much like a choke on 2m and
432. On UHF it probably looks lie a 3/4 wl antenna. Losses in the cable
probably help the match to the radio. What the heck , it works.


rain water or condensation gets into the coil in the base...

Mark

  #5  
Old November 13th 06, 08:26 PM posted to rec.radio.amateur.antenna
Norm VE3CZI
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Posts: 115
Default HOW do "dual-band"VHF/UHF antennas work?

I had a problem with an recently purchased USED Diamond SG-7500 at a Flea
Market.
Was told it was OK, but upon testing on the Vehicle I had very poor
VSWR....So much for Flea Market Bargains! !
I tested it with an MFJ Antenna Analyzer which showed it to be "out" but was
not sure whether it was the base matching unit, or the phasing elements in
the antenna rod.
I emailed Diamond and received a very nice letter with the construction of
the antenna and prices for the replacement parts. Did not solve my problem,
but "did" lead me down the right path.
I ended up borrowing a friend's SG-7500. Measured the Capacitive &
Inductive elements in the Antenna Rod..no difference. Since the base
utilizes a capacitor in series AND a coil shunt to ground, it was awkward to
evalualuate if the shunt coil or capacitor in the Base Unit was bad. I
substituted and base unit on my antenna...voila! Good Match. I ordered up
a new coil from Mfr for $35.00 and problem solved.
Substitution is sometimes the best troubleshooting technique. I suspect the
original owner put TOO much power into the Base Unit !!

73...Norm VE3CZI


"Paladin" wrote in message
ps.com...
Hello "Antenna-Lovers",
How do dual-band antennas work? I bought a COMET
"SSB-7" antenna for my mobile radio. It worked great for a year or more
and then,trouble. I don't know "HOW" to trouble-shoot it.
Where on the antenna can I check for "continuity" or somewhere I can
look to find a "fault" of sorts.
It costs a bit more than a lower gain antenna. How can I trouble-shoot
it ?

73's Paladin



  #6  
Old November 13th 06, 08:28 PM posted to rec.radio.amateur.antenna
Richard Clark
external usenet poster
 
Posts: 2,951
Default HOW do "dual-band"VHF/UHF antennas work?

On Mon, 13 Nov 2006 14:26:14 -0500, "Norm VE3CZI"
wrote:

I suspect the
original owner put TOO much power into the Base Unit !!


Hi Norm,

Did you crack that sucker open to perform an autopsy? If it is a
goner anyway, that would be useful to observe and report here too.

73's
Richard Clark, KB7QHC
  #7  
Old November 14th 06, 06:42 AM posted to rec.radio.amateur.antenna
external usenet poster
 
Posts: 137
Default HOW do "dual-band"VHF/UHF antennas work?

Did you crack that sucker open to perform an autopsy? If it is a
goner anyway, that would be useful to observe and report here too.


Paladin, I'm sure we'd all be happy to help you out if you felt like
taking the antenna apart at the base and taking a couple of photos or
describing the components... it would certainly be fruitful if you just
found a cold solder joint or something inside anyway... quick fix then.

Dan

  #8  
Old November 16th 06, 08:03 AM posted to rec.radio.amateur.antenna
Roger (K8RI)
external usenet poster
 
Posts: 52
Default HOW do "dual-band"VHF/UHF antennas work?

On 12 Nov 2006 15:11:40 -0800, "Paladin" wrote:

Hello "Antenna-Lovers",
How do dual-band antennas work? I bought a COMET
"SSB-7" antenna for my mobile radio. It worked great for a year or more
and then,trouble. I don't know "HOW" to trouble-shoot it.
Where on the antenna can I check for "continuity" or somewhere I can
look to find a "fault" of sorts.
It costs a bit more than a lower gain antenna. How can I trouble-shoot
it ?


Don't know about the Comet mobile antenna but I've been using the
Diamond series since they came out. The one on my car (4-Runner) has
hit so many trees and been straightened so many times it's beginning
to look strange. It's also worn to the point where it no longer seats
well at the fold over point. (I had to use some lapping compound on
it about two years ago to keep it from rattling). It looks like I'm
going to need to redo it at the fold over point again.

I have used the Comet duo band base station antennas with good luck
except for the one that took a direct lightning strike. Part of the
inside disappeared, the top two feet of fiberglass looked like an
exploded cigar, and the matching network at the base was shorted. The
base station antennas are well built and I'd have though the mobiles
would be the same, but apparently not.

Actually I have two Comets and one Diamond base station antennas for
144 and 440. I also have stacked pair of 11L on 440 and 12L on 144 at
130 feet. The Diamond is at 50 feet and one Comet is at 15 feet which
is the one I normally use for repeater work. The second Comet is going
to be mounted inverted under the Diamond and used for packet/jnos.

As to servicing the duo-band mobiles, there's not much to do. The
problem could be in the mount, but most likely it's either in the
molded base, or in the phasing sections. Most of these use something
that looks like a plastic insert between sections. Inside that insert
will be either a coil or a folded piece of wire that looks a lot like
a paper clip. These serve different functions on 144 and 220. Still
they either work or don't. Typically about the only thing that can go
wrong is a connection or solder joint. *Sometimes* they get some
corrosion down where the antenna fits into the base section. It is
held with one or two Allen set screws. Road salt can wreak havoc in
these areas.



73's Paladin

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com
 




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