Antenna setup, now i'm confused...
V. Wesseling
I always tought SWR was affected by coax length, and the metal surface
right below the antenna. But now most ppl here tell me it's not.
My problem is that I have my antenna at the spare tire behind my vehicle
(4x4). But the SWR is in the red zone. I can't get it down a lot by only
tweaking the antenna tip. Is SWR realy only affected by the length of
the antenna itself? What else do I have to try to get a better SWR? (The
range is not realy the issue here, I will only be using the CB on small
4x4 terrains and with 4wd autoclub days.)
With regards,
Vincent
- A Dutch Student
Mr. Know-it-all
police, and the swr police here when I reply here but oh well.
SWR is affected by all of those things you mention and others.
Length of antenna, length of coax, mounting location, physical
connections (good solder joints and tighness of connectors) and
type of antenna. If you don't want to fool with it and just want to talk
short distance, for your application, here is what I
would do. If you are happy with the mounting location on your 4x4, then
leave it there. I would try to make sure your swr is adjusted as low as
possible and just live with it. Arguements will be you will burn up your
radio
(possible but not likely) and that you are loosing all of your signal
output
(this part is true and varies with how bad your swr really is) but... if
you are doing close range transmissions usually within sight of each
other or even vehicle to vehicle like we do when up in the mountains
playing in the mud, you will be all right. You just aren't going to set
any distance records, and the possibility does exist that you could waste
the output transistors in your radio if you key down for extended time.
We have purposely tried to cook radios in the shop running them into
a bad match keyed down for a half hour or more and the failure rate
is almost non existent. Now....If you are running an amp (god forbid)
it will go Nuclear with bad swr in a real short time.....
Mr. K
PS>You didn't say what kind of antenna you are using, but if it's a whip
mounted on the tire rack, you would do better to mount it on the rear
bumper.
My recommendation would be the biggest fiberglass whip you can get your
hands
on mounted to the left rear of the bumper. Use a tie down to the windshield
when
you get in the rough stuff so it isn't whacking the hell out of everything
you go under.
V. Wesseling <vin...@dsdelft.nl> wrote in article
<3BFE4A52...@dsdelft.nl>...
V. Wesseling
> it will go Nuclear with bad swr in a real short time.....
No amp. Is not needed vor short distance :)
> PS>You didn't say what kind of antenna you are using, but if it's a whip
> mounted on the tire rack, you would do better to mount it on the rear
> bumper.
Rear bumer is plastic for most part, so no grounding :-/ Beside that, my
door opens sideways, and an antenna on the bumper would block it. Now it
turns nice and cosy with the door :)
> My recommendation would be the biggest fiberglass whip you can get your
> hands on mounted to the left rear of the bumper. Use a tie down to the windshield
> when you get in the rough stuff so it isn't whacking the hell out of everything
> you go under.
Tie it in rough is a good tip I hadn;t heared. Didn't notice others
doing it, but I'm happy you mentioned it :)
GR/EE/TZ
Vincent
Jay in the Mojave
If the antenna is mounted onto plastic, with no connection to the
vehicles body with the shield at the antenna end, this will cause a high
SWR.
Also if the antenna is mounted on top of the spare tire carrier, and has
a any length that is close to the vehicles body this will also throw off
the SWR.
The coax in this case is not the problem, unless its shield is not
connected to the vehicles body, ant the antenna end.
Jay in the Mojave
Mad Dog
SWR
versus VSWR. The only way to get a fairly accurate measurement is by using a
Electrical 1/2wavelength section of coax,( 492xVF/F). To be blunt your
Velocity factor
is probably .66 and the resonant frequency your using is 27.185, Formula-492
x .66 /
27.185 = 11.94ft. coax length.
Now that you know the deal with coax length, dont waste your time messing
with it.
The ground plane aka counterpoise aka reflector is causing your headache
because
the spare tire rack doesn,t contain enough metal to provide a counterpoise.
Try running a 8gauge braided copper wire to the frame of the vehicle from
the antenna bracket. Remember we are dealing with current flow here and
grounds are the most important. Make sure the antenna bracket is assembled
correctly with a fiber washer separating the center conductor from the
ground side. Try removing the spare tire rack
and scraping away any paint or rust that may be inhibiting your ground path.
Make sure the coax is not pinched or cut and install a chassis ground on the
case of the radio when youre all done................Good Luck
--
714 Sandpile,
The Mad Dog wavin' good bye
"V. Wesseling" <vin...@dsdelft.nl> wrote in message
news:3BFE4A52...@dsdelft.nl...
V. Wesseling
Hi,
> If the antenna is mounted onto plastic, with no connection to the
> vehicles body with the shield at the antenna end, this will cause a high
> SWR.
The tirecarrier is metal, and grounded to the vehicle. I measured the
inpedance (ohm) and that was close to 0.
> Also if the antenna is mounted on top of the spare tire carrier, and has
> a any length that is close to the vehicles body this will also throw off
> the SWR.
The entire backend top is polymer (plastic) and glass. Only metal is the
carrier, the spare wheel and the lower part of the backdoor, wich is
actualy below the antenna mounting point.
> The coax in this case is not the problem, unless its shield is not
> connected to the vehicles body, ant the antenna end.
I checked the connector today, and gained a little bit, but not the big
factor I was looking for :(
> Jay in the Mojave
Tnx,
GR/EE/TZ
Vincent
graywolf
problem is a lack of a good groundplane. I suggest that you try a CB marine
antenna. These antennas are design to operator without a groundplane.
However, they are probably not design to handle high power. Looking at
shakespears webpage it looks like there antennas are limited to 25 watts
max. Well with in the range of a legal CB radio.
http://www.shakespeare-marine.com/index.html
"V. Wesseling" <vin...@dsdelft.nl> wrote in message
news:3BFE4A52...@dsdelft.nl...
BobC
> > right below the antenna. But now most ppl here tell me it's not.
> >
-------------------------------
Mad Dog <lgs...@strato.net> wrote in message
news:9tlvrb$3hp29$1...@ID-20128.news.dfncis.de...
.....snipped...
> Now that you know the deal with coax length, dont waste your time messing
> with it.
> The ground plane aka counterpoise aka reflector is causing your headache
> because
> the spare tire rack doesn,t contain enough metal to provide a
counterpoise.
> Try running a 8gauge braided copper wire to the frame of the vehicle from
> the antenna bracket. Remember we are dealing with current flow here and
> grounds are the most important....snipped....
> --
> 714 Sandpile,
> The Mad Dog wavin' good bye
--------------------------------
For Vincent,
You are correct in knowing the antenna uses whatever metal is closest as a
groundplane. As Mad Dog pointed out, your spare tire carrier is not going to
provide much surface area and needs to be electrically attached to the
remainder of the body that the antenna can "see". If possible, attach a
short ground wire between the swinging arm of the tire carrier and the
carrier brackets. Basically you're shorting out the hinges and allowing the
vehicle body to more effectively become part of the groundplane. If the
carrier hinges are mounted in rubber then you should find a way to attach
the ground wires to those bolts having direct connection to the body.
For Mad Dog,
Your reasons for grounding are accurate enough however the method is where
we disagree. The antenna predominantly uses the metal that it can,
literally, see as the return path. The metal closest to the antenna being
the most important. The length of the ground wire is rather important and
begins to lessen its effectiveness as the length approaches a couple feet. I
figure it would need to be about that long or longer to reach any frame
members on that vehicle. Also, the frame rail isn't in the direct "view" of
the antenna and won't be carrying much rf current back from the antenna.
As you may or may not realize, the groundplane area closest to the antenna
feedpoint has the most capacitance to the antenna. The capacitance decreases
with distance away from the feedpoint. The rf current will be highest on the
metal skin closest to the antenna and decreases with distance.
BobC
Sparky
Changing the coax length will only give the 'illusion' of improving
SWR at the radio, but actually does nothing. It sounds like you have
a grounding problem. You need to mount the antenna on a part of the
vehicle that is connected to -lots- of metal like the frame, or the
body if it's all metal. The best place to mount it is the roof. If
distance isn't your thing, then a mag-mount might work best for you.
It might get knocked off once in a while, but it won't break like a
fiberglass whip.
V. Wesseling
> problem is a lack of a good groundplane. I suggest that you try a CB marine
> antenna. These antennas are design to operator without a groundplane.
> However, they are probably not design to handle high power. Looking at
> shakespears webpage it looks like there antennas are limited to 25 watts
> max. Well with in the range of a legal CB radio.
MNy CB has 4 watts, thus it should work indeed. Maybe I'll have a look
around for a marine type antenna :)
Jay in the Mojave
most CB SWR meters have the red SWR indication at 3 to 1 SWR.
more below.
"V. Wesseling" wrote:
>
> > Hello Vincent:
>
> Hi,
>
> > If the antenna is mounted onto plastic, with no connection to the
> > vehicles body with the shield at the antenna end, this will cause a high
> > SWR.
>
> The tirecarrier is metal, and grounded to the vehicle. I measured the
> inpedance (ohm) and that was close to 0.
Ok good deal, is the antenna end of the coax connected to tire carrier?
> > Also if the antenna is mounted on top of the spare tire carrier, and has
> > a any length that is close to the vehicles body this will also throw off
> > the SWR.
>
> The entire backend top is polymer (plastic) and glass. Only metal is the
> carrier, the spare wheel and the lower part of the backdoor, wich is
> actualy below the antenna mounting point.
It might have a metal mesh material to reinforce it structurally? Who
knows.
> > The coax in this case is not the problem, unless its shield is not
> > connected to the vehicles body, ant the antenna end.
>
> I checked the connector today, and gained a little bit, but not the big
> factor I was looking for :(
What did you check to get a gain? This may be a good indicator of where
the problem is.
Is there continuity between the radio end of the coax shield and the
vehicles body?, theres should be.
Is there continuity bewteen the shield and center pin, at radio end of
the coax?
Can you hear local or skip stations? Have to talked to anyone?
Jay in the Mojave
V. Wesseling
> > inpedance (ohm) and that was close to 0.
> Ok good deal, is the antenna end of the coax connected to tire carrier?
Yes, it contacts clear metal. Also 0 ohm between the carrier and the
antenna mounting bolt.
> > > Also if the antenna is mounted on top of the spare tire carrier, and has
> > > a any length that is close to the vehicles body this will also throw off
> > > the SWR.
> > The entire backend top is polymer (plastic) and glass. Only metal is the
> > carrier, the spare wheel and the lower part of the backdoor, wich is
> > actualy below the antenna mounting point.
> It might have a metal mesh material to reinforce it structurally? Who
> knows.
Could be, but then it bents realy easely ;)
> > > The coax in this case is not the problem, unless its shield is not
> > > connected to the vehicles body, ant the antenna end.
> > I checked the connector today, and gained a little bit, but not the big
> > factor I was looking for :(
> What did you check to get a gain? This may be a good indicator of where
> the problem is.
The coax was not realy good soldered to the connector. I redid that. But
as stated, here was not much gain, not the big factor i was looking for.
> Is there continuity between the radio end of the coax shield and the
> vehicles body?, theres should be.
> Is there continuity bewteen the shield and center pin, at radio end of
> the coax?
> Can you hear local or skip stations? Have to talked to anyone?
I will check that tomorrow (is now evening and realy dark today, over
here)
REceiving is realy ok, transmitting: I didn't dare to press the button
too long :-/
middle...@home.coms
guru with antenna Good Luck to you with it .. Keep up the good work Jay!
"V. Wesseling" <vin...@dsdelft.nl> wrote in message
news:3BFFCC5D...@dsdelft.nl...
SD SpecialDelivery Jones
oh why don't you buzz off.... finger, you idiot.....
Landshark
I don't normal take issue with you, as most
of the time I agree, but not this time.
He is asking very valid questions and is
in need of some assistance. He is getting
some very good answers and learning
basics of how to set up a radio, which
is a good thing.
Take Care,
Landshark
--
"I believe it was Fredrick the Great:
he who defends everything defends
nothing."
"SD SpecialDelivery Jones" <SDSpecialDe...@stalag13.com> wrote in
message news:d9f7b76c.01112...@posting.google.com...
middle...@home.coms
Are you talking to me I didn't even post in this thread. Have a Good Day
AnyHow Randall!
SD SpecialDelivery Jones
aw bullshit......you are in the thread. i just posted to the wrong
message good buddy....the point still stands........
middle...@home.coms
> message good buddy....the point still stands........
Mr. Sd, I beg to difer with you good budie.. I have not participated in this
thread until now and the post previous to this , perhaps you have me
mistaken ...come onnnnnn........ Just want to clarify any misconceptions out
there...
Bill Nelson
: I agree with the other posts. The length of cable is not an issue and your
: problem is a lack of a good groundplane. I suggest that you try a CB marine
: antenna. These antennas are design to operator without a groundplane.
: However, they are probably not design to handle high power. Looking at
: shakespears webpage it looks like there antennas are limited to 25 watts
: max. Well with in the range of a legal CB radio.
None of the responses so far have been accurate, including this one.
The coax length CAN affect the actual SWR. When? When the coax is
part of the antenna system. This is not an unusual occurance on a
mobile installation. I can just about guarantee that this is the case
for the setup being discussed. Even if you run a good ground from
the antenna base to the frame somewhere - you will still have a long
length that will be acting as even more antenna length.
So, it might be possible to reduce the SWR by fiddling with the coax
lenght. It is also possible that any changes to make it longer will only
increase the problem.
One solution is an antenna tuner - placed between the transmitter and the
long coax run to the antenna. If only using a narrow range of frequencies,
the tuner could be adjusted then left alone.
The system might not be very efficient, but that is not needed for the
short range communication being contemplated.
--
Bill Nelson (bi...@peak.org)
Tom -Doc
the 4*4 try that newer cars and trucks are doing that to save money
--
doc
http://hr2510.homeip.net:2510/
"V. Wesseling" <vin...@dsdelft.nl> wrote in message
news:3BFE4A52...@dsdelft.nl...