102" whip has higher SWR than 3ft stick?!?
kel...@imap2.asu.edu
So I got one. When parked, I replaced a 3ft FireStik (with a SWR of 1.1:1
on the channel I've tuned it to) with the whip. Shoom! The SWR zips up to
the 2 to 2+ range! This, with a mag mount in the center of the cab. What
gives?
I've also read that the 102" whip is king even when the SWR is higher.
There's a fellow in Montana, I believe, who posts here, that experimented
with the whip on the bumper and a loaded antenna on the roof. Better SWR
on the roof, better performance on the bumper!
Does this mean one should ignore moderately-crappy SWR if it's a whip?
And second question: Does the 102" whip need a spring under it? Ie, does
the spring become part of the antenna? (I tried it both with and without
spring, and the SWR was a little better with the spring, if I remember
correctly.
Thanks for replies,
-------------------------------------------------------------------
Kelton Rhoads kel...@asu.edu 602/350-9462
Dept of Social Psychology, Az State Univ
-------------------------------------------------------------------
Joe Fitter BV/N0IAT
antennas:
1. Assuming CB channel 19 (27.185), the correct length for a 1/4 wave
antenna is 103.3 inches. The spring _does_ infact become part of
the radiating element, hence altering the VSWR slightly and showing
a less than ideal match (at least according to the swr meter).
Perhaps you are measuring the VSWR at channel 1, without the spring,
so I'd expect the VSWR to be about 1.5:1 or so.
2. More likely than not, the IMPEDANCE of the antenna changes
when converting from a 102 inch whip to a 3 foot loaded whip. The
difference in impedance matching in a mobile environment is usually
caused by the ground-plane. I suspect the ground plane is not ideal,
which is causing an impedance "bump". This could be caused by
a less-than ideal antenna mount, or simply a "poor mounting location"
on the vehicle.
While I've not used CB radio's in a long time, what Chris in Montana says
is basically true....that is, the longer radiator will usually radiate
more efficiently that a perfectly matched short/loaded radiator. Some
rigs today have high-VSWR protection circuits that cut the radio output
power back when VSWR rises above 3:1. In this case, if the VSWR on the
WHIP is higher than 3:1, your actual output power could be cut back
electronically and the result would be better performance on the little
loaded 3 ft whip compared to the 102 inch antenna. Otherwise, the 102
inch should win in every competition assuming same-mounting location on
the vehicle.
As for using a spring or not, you might find the spring provides a
slightly better match on lower channels (1 thru 15, for example). The
surface area of the spring may also create some additional capacitance
between ground and radiator too...this could improve the match, lowering
the VSWR even on higher channels (30 - 40).
If you are an "SWR JUNKIE", who only likes to see a perfect 1:1 match, I
suggest using a capacitor or two to mount between the antenna ground and
the antenna radiator...this usually improves matching. I've used this
approach for several years in mobile amateur radio installations with
good results. Send me email and I can advise further.
Oh yea...I assume the coaxial cable, coax PL259, and antenna mount were
the "same" in your 3ft vs 8 ft whip comparison. If not, the high VSWR
might be due to a bad cable or connector.
Joe
----------------------------------------------------------
Amateur Radio: BV/N0IAT Taipei TAIWAN Republic of China
ex. 7J1AOF (Japan) YU3/N0IAT (Slovenia) KA0ZDH (Novice)
Licensed Radio Amateur since 1986. Comments are mine only.
----------------------------------------------------------
Frank Scaraglino
the low end, the antenna i stoo long. If it gets better at the high end,
the antenna is too short. Also a 102" whip is better because there is
"more metal" in the air!! Frank
Joe Buch
>I've heard frequently that you just can't beat a 102" whip on a mobile.
>So I got one. When parked, I replaced a 3ft FireStik (with a SWR of 1.1:1
>on the channel I've tuned it to) with the whip. Shoom! The SWR zips up to
>the 2 to 2+ range! This, with a mag mount in the center of the cab. What
>gives?
The impedance of any antenna is made up from two components,
the loss resistance and the radiation resistance. In a 1/4
wave whip (~102 inches at 27 MHz), mounted on a vehicle, the
impedance is about 25 ohms. That is about 1 ohm of loss
resistance and 24 ohms of radiation resistance. A 25 ohm
impedance presents a 2:1 SWR to a 50 ohm coax.
A 3 foot helical whip will have a much lower radiation
resistance and a much higher loss resistance. The loss
comes from the long length of small diameter wire used to
wind the helical antenna. If the loss is 40 ohms and the
radiation resistance is 10 ohms, that will appear to be a
perfect 50 load on your coax. A 1:1 SWR results.
The efficiency of any antenna is:
efficiency= radiation resistance/(radiation res.+ loss res)
102" whip efficiency = 24/25 = 96%
36" helical efficiency = 10/50 = 20%
The difference is about 6.8 dB or one S unit in favor of the
longer antenna. (Note: The numbers chosen here are typical
and are very dependent on how the antenna is installed on
the vehicle. Your results may vary.)
So that is why a longer antenna works better even though the
SWR is worse.
~*-.,_,.-*~'^'~*-.,_,.-*~'^'~*-.,_,.-*~'
Joe Buch N2JB
Editor, NASWA Journal Technical Topics
bu...@exis.net
-*~'^'~*-.,_,.-*~'^'~*-.,_,.-*~'^'~*-.,_
Bryan
The 102" bumper mounted whip on my Ranger always measure very low SWR.
Usually 1.1 or 1.2 to 1 (indicated). This is with a full size spring.
Also, I was 'religious' in preparing the coax, terminals, connector,
weatherproofing, etc. A good RF ground is needed at the mounting point.
You need the spring. I would expect that you should be seeing 1.5 to 1
for a 1/4 wave whip.
When using a short center-loaded antenna on top of the cab with a magnetic
mount, the SWR is much higher. Roughly 1.8 to 1 at the edge of the band, and
1.4 in the center. The short, loaded antenna has narrow bandwidth as one
would expect.
Keep in mind that most consumer grade SWR meters are not precision
instruments. They'll give you a reasonable estimation of your antenna
system's relative SWR. Usually, the actual SWR will be somewhat higher
than the reading indicated.
Comparing performance, nothing will come close to a 1/4 wave whip. The only
gotcha is that the bumper mounted 1/4 wave is not omnidirectional, generally
considered a disadvantage for mobile operation. It's great for staionary
DX use, though.
Best of Luck
Bryan
mail...@pop.net
> kel...@imap2.asu.edu writes:
...snip
Jeff East
> Comparing performance, nothing will come close to a 1/4 wave whip. The only
> gotcha is that the bumper mounted 1/4 wave is not omnidirectional, generally
Assuming its on the rear bumper, what direction does it favour?
Cardioid pattern with a read null?
Bob Archer N4ECO
The best direction will be across the longest piece of metal
of the vehicle.
And the null will be greatest where there is no vehicle metal.
>==========Jeff East, 2/14/96==========
Bob Archer N4ECO
Joe Fitter BV/N0IAT
> Assuming its on the rear bumper, what direction does it favour?
>Cardioid pattern with a read null?
Yes. Basically correct if you meant cardioid with a *rear* null as
follows (maybe just a type error, no?):
______________ z
:x
Front of car 0 VIEWED FM :
TOP OF :
0 CAR ROOF :
Y ______________
Y= direction of maximum field strength
z= direction of minimum field strength
x= rear bumper mounted whip antenna
Of course, this varies from car to car, but generally the signal
strength (field strenght) is strongest in the direction of maximum
"counterpoise".
Chad Payne
> There's a fellow in Montana, I believe, who posts here, that experimented
> with the whip on the bumper and a loaded antenna on the roof. Better SWR
> on the roof, better performance on the bumper!
Than fellow would be me.
> Does this mean one should ignore moderately-crappy SWR if it's a whip?
> And second question: Does the 102" whip need a spring under it? Ie, does
> the spring become part of the antenna? (I tried it both with and without
> spring, and the SWR was a little better with the spring, if I remember
> correctly.
I put up with an SWR of 2:1 with my 102" whip and it works great. As for
the spring question, yes, use a spring. It does become part of the
antenna and you need it. If you are using the regular CB frequencies
then an acutal quarter wave is around 108". If you add a 6" spring then
you have the correct antenna length.
\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/
Chad Payne | "Remember, bananas are good for your baud!"
Operations Director | (406)543-1928 - voice ban...@split.com
Banana Programming | (406)549-3522 - Fax http://www.split.com
Missoula, Montana | (406)543-8234 - BBS ftp.split.com
/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/-\-/
Rich Mulvey
>How much coax are you using? 20ft. 18ft. 12ft. ???
>it does make a difference.
Could you explain the physics behind that, please?
Chad Payne
> (say 6") off an old antenna and solder it to the top of the 102"
> antenna to creat the 108" 1/4 wave? That way we can eliminate
> the need to get a spring.
>
> What do you think?
Yes, mechanically that would work fine, but the spring is always a good
idea as it gives the antenna more flexability if you hit something overhead,
like a tree or a low flying aircraft. I use my spring when I bend my
antenna to fit my truck into the garage.
> P.S. I want to try to place the 102" + 6" on my bike while I
> do a little bike riding. The antenna will be mounted on
> the aluminum bike rack at the rear. Do you think that
> I will have to make a ground plane radial for it to work
> ok? I just don't want to have 4' radial sticking out the
> sides of my bike, could hurt a pedestrian.
You won't have much of a ground plane so I doubt your signal will be as
good as if you mounted the 102" whip on a larger vehicle. They sell
these little mini ground planes that fit at the bottom of a standard CB
antenna. They have three 4" (yes that is inch) radials. I'm not sure
how well they work, but it is worth a try.
> The highly visible flag shouldn't harm the signal, I hope.
> Can you imagine if I hit a bump? The whip is going to
> fly here and there. I just hope it doesn't hit me in
> the back too hard.
This could be a problem. Unless there is something between you and the
antenna I'd watch out. They do flop around a lot.
Chad
5 Mary Tango 127
Western Montana
Paul Lopes
it does make a difference.
Paul
iro...@localnet.com
>On 16 Feb 1996 04:00:03 GMT, Paul Lopes <PLo...@worldnet.att.net> wrote:
>>How much coax are you using? 20ft. 18ft. 12ft. ???
>>it does make a difference.
> Could you explain the physics behind that, please?
THE ONLY TIME COAX LENGTH HAS ANYTHING TO DO WITH YOUR SWR IS WHEN YOU
ARE USING A MAGNET MOUNT ANTENNA, THIS IS NOT TO SAY THAT IT AFFECTS
YOUR ACTUAL MATCH HOWEVER JUST THAT IT AFFECT HOW YOUR SWR METER WILL
READ IT AT THE RADIO'S END.
**********COAX LENGTH WILL NEVER AFFECT YOUR VSWR*************
ALWAYS CHECK SWR AT THE ANTENNA IF YOU WANT THE MOST ACCURATE RESULTS,
PERIOD.
P.S. I'M YELLING BECAUSE THE SHIT IS GETTING DEAP AND MY BOOTS ARE
FILLING UP.
73'S
KB2WCK
DR"T"@wwa.com
> How much coax are you using? 20ft. 18ft. 12ft. ???
> it does make a difference.
>
>
>
>
>
>>>>
The only reason it makes a difference is, your antenna is not tuned.
The antenna should be tuned at the base or feed point
for the lowest SWR and then any length of coax will do.
If you want to prove this, cut any length of coax and place
a 50 ohm dummy load at one end check the SWR, it will be
1:1.......end of thread.
"73" DR"T"
Val Breault
> On 16 Feb 1996 04:00:03 GMT, Paul Lopes <PLo...@worldnet.att.net> wrote:
> >How much coax are you using? 20ft. 18ft. 12ft. ???
> >it does make a difference.
>
> Could you explain the physics behind that, please?
I'll field that one.
We want to maximize the amount of power the antenna radiates and
minimize the amount of reflected power so we measure the SWR and
adjust the antenna system to minimize it.
SWR is a measure of the power that was not radiated by the antenna.
The power has to go somewhere so, like ripples on a pond where they
meet a pier, it reflects from the antenna and travels back down the
feedline.
ALL coax is lossy. Some coax has a higher loss coefficient than other
coax but they all lose some amount of signal and the amount of loss is
proportional to the length of the coax. A longer feedline will lose
more power than a short feedline. This loss is bidirectional. A long
feedline will attenuate the signal going to the antenna and (this is
the answer to the question) also the reflected power. Lower reflected
power=lower SWR.
If you had a piece of coax that was long enough you could dissipate
all your signal in the feedline without an antenna on the other end.
Yep, you could short circuit the far end or leave it disconnected and
not see any difference at the radio end of the line. It would be an
example of a perfect SWR with ZERO radiated signal.
Something for the nit pickers to feed on:
For non-resonant antennas a matching transformer can be affected from
a specific length of feedline. If an antenna system is unusually
sensitive to the length of its feedline then it's probably
non-resonant through design or execution. As far as I know all
consumer grade CB antennas are intended to be resonant at the terminal
connection.
--
Val Breault vbre...@gmr.com \ /| ars N8OEF
General Motors R&D Center, Warren, MI \ / | pres GM ARC
My opinions are not necessarily those \ /__| HF mobile on
of GM R&D or of the GM Corporation. \/ |___ 14.200 +/-
Rich Mulvey
>In article <slrn4ia8pu....@ll.aa2ys.ampr.org> mul...@ll.aa2ys.ampr.org (Rich Mulvey) writes:
>> On 16 Feb 1996 04:00:03 GMT, Paul Lopes <PLo...@worldnet.att.net> wrote:
>
>> >How much coax are you using? 20ft. 18ft. 12ft. ???
>> >it does make a difference.
>>
>> Could you explain the physics behind that, please?
>
>
>coax but they all lose some amount of signal and the amount of loss is
>proportional to the length of the coax. A longer feedline will lose
>more power than a short feedline. This loss is bidirectional. A long
>feedline will attenuate the signal going to the antenna and (this is
>the answer to the question) also the reflected power. Lower reflected
>power=lower SWR.
>
>If you had a piece of coax that was long enough you could dissipate
>all your signal in the feedline without an antenna on the other end.
>Yep, you could short circuit the far end or leave it disconnected and
>not see any difference at the radio end of the line. It would be an
>example of a perfect SWR with ZERO radiated signal.
>
My comment obviously wasn't clear enough. ;-)
The person who was initially asking about feedline length was obviously
falling prey to the oft-repeated fallacy in rec.radio.cb that
the feedline length must be a multiple of some magic number
( usually 3' is quoted ), or the SWR would be un-matchable.
While your comments are perfectly correct, the amount of
attenuation from several feet of coax in a mobile installation at 11M
will be unmeasurable by the consumer-grade SWR meter used by the
typical CB'r or ham. Assuming, of course, that the antenna/feedline
combination isn't being used as a random-wire. :-)
- Rich
---
Rich Mulvey, aa2ys Rochester, NY USA
mul...@vivanet.com
aa...@net.wb2psi.ampr.org
aa2ys@wb2psi.#wny.ny.us
David Harrell
I am an owner of both types of antennas. I have a 102" steel whip
which I bought a Radio Shack and a stock type of 3" top loader. I am an
avid DX operator on the 11 meter freeband and have made contacts on both.
I have made more contacts on the 102". They are both great antennas. I
seem to believe that a 1/4 wave is better because of it's size. I
deffinately do agree with you. I do not have my 102" on a car. I have it
hooked up on my metal shed in my backyard. It has a good ground. It has
never failed me when the propagation gets started. The top loader has
never failed me either except whenthe skip is going in a different
direction than my ground. My 102" is taller and is a perfect omni
directional antenna for having it grounded in a south ward direction. I
hope my general information helps a few readers.
73's
Jeremy 2SEO143
Virginia
ZANDOR1
Ronald Shapiro
to the bottom of the spring
Does the 102" whip need a spring under it? Ie, does
>> the spring become part of the antenna?
Ron, in Hyde Park, New York.......EZEB86A@prodigy.com
Bill Nelson
: What is the difference in the length of coax???
Well, if it is too short, it won't reach from the rig to the antenna.
Bill
Paul Di Lorenzo
That has to be the funniest think I have heard in a long time!
Coax wont reach the radio Ha! Ha!.. For all those HAM's out there
Hi! Hi!.....FUNNY......Keep up the great work Bill. I love it!!
Thanks
Paul WB2IMT..
linv...@freenet.edmonton.ab.ca
: Bill
Yes, that would increase the SWR to unacceptable readings. Not even the
best antenna tuner could be of any help.
Glenn Thomas
resistance of a full quarterwave (108") whip is much higher than a shorter
antenna. At an exact quarterwave the feedpoint impedance will be around
25 ohms resistive. If the whip is much shorter or longer, impedence will
be reactive. If you REALLY think you need a 1:1 SWR, pick an antenna of
any length (the radiation resistance and hence the effectivness of the
antenna will be effected by this, so choose carefully!), measure or
figure out what its complex impedence is and build yourself a matching
network to transform the actual antenna impedence to 50 ohms resistive.
You now have an antenna that is as effective as its radiation resistance
and environment will let it be and your radio will see a 1:1 VSWR.
In fact, the VSWR on your coax will be 1:1 (if the matching network is
at the antenna) no matter what the antenna itself really is. This is
how antenna manufacturers design their products, they find clever ways
to design that matching network (loading coils, capacitive hats,
autotransformers etc.) so that your radio will see 1:1 no matter how
effective or ineffective the antenna actually is.
One more thing for folks who think that VSWR has anything directly to
do will antenna effectivness. I have a very nice dummy load here. I can
connect any length of 50 ohm coax to it and always have a 1:1 SWR.
However good its VSWR, it's a lousy antenna for transmitting!
73 de Glenn
Codys PC
>: What is the difference in the length of coax???
>
>Well, if it is too short, it won't reach from the rig to the antenna.
>
>Bill
;-)!
A note from the LITTERBOX.TXT...
... BUFFERIN = Relieves operator headaches casued by small BUFFERS
Frank PoWell
says...
>
>mul...@ll.aa2ys.ampr.org (Rich Mulvey) wrote:
>
>>On 16 Feb 1996 04:00:03 GMT, Paul Lopes <PLo...@worldnet.at
>t.net> wrote:
>>>How much coax are you using? 20ft. 18ft. 12ft. ???
>>>it does make a difference.
>
>> Could you explain the physics behind that, please?
>
>S WHEN YOU
>ARE USING A MAGNET MOUNT ANTENNA, THIS IS NOT TO SAY THAT I
>T AFFECTS
>YOUR ACTUAL MATCH HOWEVER JUST THAT IT AFFECT HOW YOUR SWR M
>ETER WILL
>READ IT AT THE RADIO'S END.
>
>**********COAX LENGTH WILL NEVER AFFECT YOUR VSWR***********
>**
>ALWAYS CHECK SWR AT THE ANTENNA IF YOU WANT THE MOST ACCURAT
>E RESULTS,
>PERIOD.
>
>P.S. I'M YELLING BECAUSE THE SHIT IS GETTING DEAP AND MY BO
>OTS ARE
>FILLING UP.
>
>73'S
>
>KB2WCK
>
>
>
the length of the coax has nothing to do with the antenna impedance, just
the tricked impedance at the transmitter.
--
/\_/\ I never met a cat |\ /| | Frank PoWell - N7KSK -
( o.o ) I didn't like! O.O | lkil...@xmission.com
\ - / ___,,,^..^,,,___ =(___)= |
/ \ ftp://ftp.xmission.com/pub/users/l/lkilgore/sources.html