VHF Antenna Question
This is to ask if anyone might have an explanation for something very odd happening with a couple of VHF antennas on my boat. What's happening mystifies me and my electronics tech--it ain't supposed to happen but it does!
The boat has a Digital Antenna model 532 VW-R 16-foot 10 DB gain antenna that does not function properly. It is mounted on the cabin top of my Nordhavn 47. A few feet away is a DA 8-foot 6 DB gain antenna also mounted on the cabin top at the same exact level. The antennas are mounted so there is no shadowing from one another or any other antenna or structure. The two antennas feed identical Standard Horizon QUEST VHF radios.
The problem is that the 6DB gain antenna outperforms the 10 DB gain antenna by a wide margin.
To confirm this, my dealer and I swapped antenna leads--no matter which radio we use, the 6
outperforms the 9 by about 50% when bringing in distant stations.
Based on that, it appeared to me that something had gone wrong with the 10 DB gain antenna,
so we obtained a brand Digital Antenna model 532 WV-R 9 DB gain antenna and tried it. The PL-259
(end fitting) used was the one furnished by the manufacturer with the antenna, and we ran the cable
straight from the antenna to both radios with no additional cables or connections. We had identical
results: the 6 DB gain antenna outperformed the taller 9 DB gain antenna by 50%, as judged by
bringing in a weak station--we used a NOAA weather station.
Neither my technician nor I has an explanation for this. Clearly, the taller, higher-gain antenna
should outperform the shorter, lower-gain one. But here we have two cases where that is not
happening.
I spec'd Digital Antenna brand antennas because I felt they make the best. Now I am thinking it't
time to go back to Shakespeare, whom I used for many years--especially so, since I cannot get a
anyone from Digital Antenna on the phone to talk to me about this. Also, no e-mail response to
a request for advice left on their website.
Any ideas what's happening here? My best explanation is some kind of systemic problem with the
10 DB antennas.
Many thanks for your ideas,
--Milt Baker
Nordhavn 47 Bluewater
954-610-5193
I can't answer your question but what I know, is the gain in antenna is for
transmitting, not for reception.
The higher the gain is, the flatter the donut is so it concentrates emission
power, not reception.
That's all I can say about your request.
Sylvain Sirois
http://www.plongeess.com
-----Message d'origine-----
De : trawlers-and-trawlering-bounces+ssirois=cgocable.ca@lists.samurai.com
[mailto:trawlers-and-trawlering-
bounces+ssirois=cgocable.ca@lists.samurai.com] De la part de Milt Baker
Envoyi : 8 fivrier 2007 10:35
@ : TrawlerWorld List - Post Message
Objet : T&T: VHF Antenna Question
This is to ask if anyone might have an explanation for something very odd
happening with a couple of VHF antennas on my boat. What's happening
mystifies me and my electronics tech--it ain't supposed to happen but it
does!
The boat has a Digital Antenna model 532 VW-R 16-foot 10 DB gain antenna
that does not function properly. It is mounted on the cabin top of my
Nordhavn 47. A few feet away is a DA 8-foot 6 DB gain antenna also
mounted on the cabin top at the same exact level. The antennas are
mounted so there is no shadowing from one another or any other antenna or
structure. The two antennas feed identical Standard Horizon QUEST VHF
radios.
The problem is that the 6DB gain antenna outperforms the 10 DB gain
antenna by a wide margin.
To confirm this, my dealer and I swapped antenna leads--no matter which
radio we use, the 6
outperforms the 9 by about 50% when bringing in distant stations.
Based on that, it appeared to me that something had gone wrong with the 10
DB gain antenna,
so we obtained a brand Digital Antenna model 532 WV-R 9 DB gain antenna
and tried it. The PL-259
(end fitting) used was the one furnished by the manufacturer with the
antenna, and we ran the cable
straight from the antenna to both radios with no additional cables or
connections. We had identical
results: the 6 DB gain antenna outperformed the taller 9 DB gain antenna
by 50%, as judged by
bringing in a weak station--we used a NOAA weather station.
Neither my technician nor I has an explanation for this. Clearly, the
taller, higher-gain antenna
should outperform the shorter, lower-gain one. But here we have two cases
where that is not
happening.
I spec'd Digital Antenna brand antennas because I felt they make the best.
Now I am thinking it't
time to go back to Shakespeare, whom I used for many years--especially so,
since I cannot get a
anyone from Digital Antenna on the phone to talk to me about this. Also,
no e-mail response to
a request for advice left on their website.
Any ideas what's happening here? My best explanation is some kind of
systemic problem with the
10 DB antennas.
Many thanks for your ideas,
--Milt Baker
Nordhavn 47 Bluewater
954-610-5193
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I think your answer can be found below in this excerpt from a boatus
article.
"The law against free lunch applies to antennas. The greater the gain of the
antenna the more directional it becomes. Everything will be fine so long as
a 6 or 9 db antenna is vertical and stable. However, as the vessel rolls and
pitches the antenna will no longer be vertical. The signal radiated from the
more directional 6 or 9 db antenna may then be directed upward into the sky
or downward into the surrounding sea, rather than in the direction of the
horizon. A good general rule for antenna selection is to use a 3 db antenna
for sailboats and either a 3 or 6 db antenna for power boats. In general, 9
db antennas should be reserved for use on land. The directional
characteristics of the antenna operate in the receive mode as well as when
the antenna is used for reception of incoming signals."
Full article can be found at this link:
http://www.boatus.com/husick/c_antenna.asp
Greg Steckel
M/V Different Drummer
President 35 Sundeck
Frog Mortar Creek, MD
www.chesapeakebayboater.com
www.fmyc.org
----- Original Message -----
From: Milt Baker
To: TrawlerWorld List - Post Message
Sent: Thursday, February 08, 2007 10:35 AM
Subject: T&T: VHF Antenna Question
This is to ask if anyone might have an explanation for something very odd
happening with a couple of VHF antennas on my boat. What's happening
mystifies me and my electronics tech--it ain't supposed to happen but it
does!
The boat has a Digital Antenna model 532 VW-R 16-foot 10 DB gain antenna
that does not function properly. It is mounted on the cabin top of my
Nordhavn 47. A few feet away is a DA 8-foot 6 DB gain antenna also mounted
on the cabin top at the same exact level. The antennas are mounted so there
is no shadowing from one another or any other antenna or structure. The two
antennas feed identical Standard Horizon QUEST VHF radios.
The problem is that the 6DB gain antenna outperforms the 10 DB gain
antenna by a wide margin.
To confirm this, my dealer and I swapped antenna leads--no matter which
radio we use, the 6
outperforms the 9 by about 50% when bringing in distant stations.
Based on that, it appeared to me that something had gone wrong with the 10
DB gain antenna,
so we obtained a brand Digital Antenna model 532 WV-R 9 DB gain antenna
and tried it. The PL-259
(end fitting) used was the one furnished by the manufacturer with the
antenna, and we ran the cable
straight from the antenna to both radios with no additional cables or
connections. We had identical
results: the 6 DB gain antenna outperformed the taller 9 DB gain antenna
by 50%, as judged by
bringing in a weak station--we used a NOAA weather station.
Neither my technician nor I has an explanation for this. Clearly, the
taller, higher-gain antenna
should outperform the shorter, lower-gain one. But here we have two cases
where that is not
happening.
I spec'd Digital Antenna brand antennas because I felt they make the best.
Now I am thinking it't
time to go back to Shakespeare, whom I used for many years--especially so,
since I cannot get a
anyone from Digital Antenna on the phone to talk to me about this. Also,
no e-mail response to
a request for advice left on their website.
Any ideas what's happening here? My best explanation is some kind of
systemic problem with the
10 DB antennas.
Many thanks for your ideas,
--Milt Baker
Nordhavn 47 Bluewater
954-610-5193
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The problem is that the 6DB gain antenna outperforms the 10
DB gain antenna by a wide margin...
You described swapping antennas and finding that the 6dB antenna still
outperformed the 10dB one. But did you also switch cables?
There are 2 important considerations with VHF cables. The first is the line
loss of the cable itself. Different types of cables have much different
loss characteristics. The line loss of a VHF cable itself can make way more
than a 4dB difference, especially if poor quality cables are used.
The second consideration is the cable length. Line loss is linearly related
to cable length. Double the length and you've doubled the loss. Again,
that loss can be considerable. An additional problem with extra cable left
over is that we all tend to coil it up. Coiling the cable can create a
variety of RF characteristic changes that can also affect signal loss.
Before doing anything with the antennas themselves, make sure you've
experimented with swapping cables too. If you have done that then it sounds
like something is wrong with the 10dB antenna.
...Jeff
---=
Jeffrey Siegel
M/V aCappella
DeFever 53PH
W1ACA/WDB4350
Castine, Maine
www.activecaptain.com
Content, Communications, Community
Milt wrote:
This is to ask if anyone might have an explanation for something very odd
happening with a couple of VHF antennas on my boat. What's happening
mystifies me and my electronics tech--it ain't supposed to happen but it
does!
The boat has a Digital Antenna model 532 VW-R 16-foot 10 DB gain antenna
that does not function properly. It is mounted on the cabin top of my
Nordhavn 47. A few feet away is a DA 8-foot 6 DB gain antenna also mounted
on the cabin top at the same exact level. The antennas are mounted so there
is no shadowing from one another or any other antenna or structure. The two
antennas feed identical Standard Horizon QUEST VHF radios.
The problem is that the 6DB gain antenna outperforms the 10 DB gain antenna
by a wide margin.
To confirm this, my dealer and I swapped antenna leads--no matter which
radio we use, the 6
outperforms the 9 by about 50% when bringing in distant stations.
Reply:
I think that the mystery is in part antenna design and in part manufacturer
gamesmanship. FWIW, I have three different VHF antennas aboard, a Morad 10
db, a Morad 6 db, and a homemade 1/4 wave ground plane that should be 3 db.
There is very little difference in reception and we have been told that the
homemade antenna sounds the best when we talk. I have swapped radios more
times than you can imagine with each antenna because I was having trouble
with the Icom 502 squawking when we transmitted on the other radios.
In order to get extra gain from a vertical (omni-directional) antenna,
Digital Antenna and others build what is called a collinear phased antenna.
This is basically a bunch of little antennas stacked on top of each other.
The theory behind stacking antennas is that the transmission and reception
is like going through an electrical Venetian blind. The stronger signals
get in and out in the direction that you want them to go and not up or down
where they do not do any good. On reception, stacked antennas should
multiply the signal received. The trouble is that a collinear phased
antenna (a type of stacked antenna) acts like a bunch of little antennas
upon transmission (that's good) but acts like a long wire on reception
(that's bad). Naturally, the manufacturers use the gain numbers relative to
transmission to sell you on the big one. Here is a little article that I
found that explains it a lot better than I can:
http://www.popularwireless.com/gmrsrpt051599.html
IMHO, the 10 db antennas are not worth the extra bucks. The Morad 6 db is a
good sturdy antenna but I do not believe the 6 db claim. 6 db gain over
what? Transmitting through a dead chicken? It is just a 5/8 wave whip
antenna. The relative gain that manufacturers use is another game that they
play but you would hope that Digital Antenna would at least use the same
baseline for measuring their antennas. Shakespeare antennas look pretty and
most of what you pay for is the fiberglass radome. There is not much
inside. Which came first, their antennas or their fishing rods anyway?
If you look at 2 meter ham antennas you see that manufacturers can build a
darn good antenna for a lot less than what boaters usually pay for a VHF
antenna. Why do they always stick it to us boaters...
Eric Thoman
Abyssinia