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Message |
    
852
Advanced Member
Username: 852
Post Number: 585
Registered: 3-2006
| | Posted on Tuesday, October 28, 2014 - 7:16 pm: | 
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My new truck came prewired for dual antennas. My question is what length coax should be running to each side? It appears from that from the factory the coax going to each antenna seems to be between 6 to 9 feet in length. It is RG59 75ohm coax. Also how do I go about adjusting the SWR's on dual antennas? |
Dale
Senior Member
Username: Dale
Post Number: 2130
Registered: 12-2002
| | Posted on Wednesday, October 29, 2014 - 8:10 pm: |
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usually co phased coax comes in 9 or 18 feet
from what ive seen. you,ll need a 18 feet
lenth of 50 ohm coax and adjust 1 antenna at
a time.then hook up both antennas using the
75 ohm coax and swr SHOULD be same [close]
now the bad part is after all the hassle only
benifit ive seen is when driving on interstate
talking to drivers behind me or in front of me
i did SLIGHTLY better.in most cases id say just
use 1 antenna.
dale/a.k.a.hotrod
cef426
cvc#64
454 [dx numbers]
38lsb
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Sonoma
Intermediate Member
Username: Sonoma
Post Number: 304
Registered: 11-2001
| | Posted on Wednesday, October 29, 2014 - 10:44 pm: |
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Dale forgot to tell you if you run just one antenna you will have to run a piece of 50 ohm coax. the stock setup is 75 ohm which is used for 2 antennas.most of the drivers I deal with go to one antenna so they can get the swr as low as possible. duals if not far enough apart is hard to get the swr below 2.0. either way should work just fine for you. just trial and error to see which works the best for you |
Milkman21218
Advanced Member
Username: Milkman21218
Post Number: 606
Registered: 1-2004
| | Posted on Wednesday, October 29, 2014 - 11:21 pm: |
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I took one antenna off and replaced it with a dummy load. Tune the antenna. Then repeat for the other antenna.
Izzy
CEF#502
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Dale
Senior Member
Username: Dale
Post Number: 2132
Registered: 12-2002
| | Posted on Thursday, October 30, 2014 - 7:54 pm: |
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sonoma ive done duals many years ago . i was giving 852 help in setting them up. my swr was
always around a 1.5.1 or slightly lower which is fine . milkman never tried that way. been several
years since ive dione a dual setup
dale/a.k.a.hotrod
cef426
cvc#64
454 [dx numbers]
38lsb
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Sonoma
Intermediate Member
Username: Sonoma
Post Number: 305
Registered: 11-2001
| | Posted on Friday, October 31, 2014 - 11:45 am: |
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i did SLIGHTLY better.in most cases id say just
use 1 antenna.
Dale just wanted him to know if he uses one antenna to put a length of 50 ohm coax. you would be surprised at how many guys have came in with one antenna on a truck and complaining about not able to get out and said they had been told to run one antenna and they just left the dual setup on the mirrors and took one antenna off. they would be running one length to a mount without a antenna in it. your way to setup duals is how I have done it for years myself. have seen the driver just unplug one leg of the coax at the back of the radio and left the y pig tail in place and the open end just bouncing around in the over head. |
Dale
Senior Member
Username: Dale
Post Number: 2133
Registered: 12-2002
| | Posted on Saturday, November 01, 2014 - 9:04 am: |
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i agree. ive seen it too. the only bebefit i
seen was when on interstate talking to truckers
the duals did do better. but it wasnt day and
nite difference.if someone was only allowed a
stock 4 watt cb in thier rig then just maybe
id reccomend setting up a dual setup.talkibng
to base stations i couldnt see any differences
if there was any u couldnt hear or see it
dale/a.k.a.hotrod
cef426
cvc#64
454 [dx numbers]
38lsb
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Scrapiron63
Advanced Member
Username: Scrapiron63
Post Number: 936
Registered: 12-2001
| | Posted on Saturday, November 01, 2014 - 4:39 pm: |
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Two antennas just looks cool on those big shiny trucks. ;) |
852
Advanced Member
Username: 852
Post Number: 586
Registered: 3-2006
| | Posted on Saturday, November 01, 2014 - 5:18 pm: |
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I had heard to set them up as you speak of, but have also heard to set them up with the 75 ohm coax already connected. Wilson says to do it as you are saying, but Firestik says to adjust them with the 75ohm dual coax, and when adjusting one to adjust the other the same amount and recheck after each time.. |
Sonoma
Intermediate Member
Username: Sonoma
Post Number: 306
Registered: 11-2001
| | Posted on Saturday, November 01, 2014 - 9:51 pm: |
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852 you can do it either way but Dale's way is normally the easiest.I have done about 6 or 700 trucks in the last 15 years and I use a antenna analyzer to set them up and doing them the way Dale said gets you there faster than trying to tune them both at the same time. it is too much trial and error doing both at the same time for me. |
852
Advanced Member
Username: 852
Post Number: 588
Registered: 3-2006
| | Posted on Sunday, November 02, 2014 - 7:36 am: |
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Okay Thanks for the replies.... |
Tech237
Moderator
Username: Tech237
Post Number: 1821
Registered: 4-2004
| | Posted on Sunday, November 02, 2014 - 4:15 pm: |
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to get any real benefit from dual antennas they need to be around 9ft apart, anything less and you're wasting time and effort.
Spacing is as important as proper phasing.. Get the wrong spacing or phasing and you'r signal is not going to go where you wanted it. In fact get it far enough wrong and you could end up with a null where you wanted signal.
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Dale
Senior Member
Username: Dale
Post Number: 2134
Registered: 12-2002
| | Posted on Tuesday, November 04, 2014 - 5:44 pm: |
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wow sonoma i didntb yet have my antenna anylizer
way back then. i bet it be alot easier with
anylizer.tech 237 i do agree with you on spacing.
which is why i usually say go single. but yrs ago
on my 1984 chevy p/u i had dual wilson silverloads
now that truck isnt anywhere near 9 ft wide .
but i did see a tiny bit of difference in favor
of the duals .but it was only vehicles behind me or in front of me. spent nearly 5 hrs driving up/down interstate with 2 other vehicles one behind one in front.then agin with
them on oppisite side. none of us expected
anything in fact i was told i was wasting my time, but they saw a tiny difference and so did i. to this day im surprised and seen truckers still use duals on a few installs
dale/a.k.a.hotrod
cef426
cvc#64
454 [dx numbers]
38lsb
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Ke0koy
Junior Member
Username: Ke0koy
Post Number: 34
Registered: 12-2018
| | Posted on Wednesday, June 02, 2021 - 11:55 am: |
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I am going to be helping a friend tune up a set of dual whips on his semi in a month or so. I have yet to go look at the truck in person. I am looking for advice on the topic as I have never attempted a dual setup before. That said, I am no stranger to tuning single antennas. I have the basics covered, just hoping someone can share their experience on a mutual coupling issue I am hung up on.
What is already set up: The whips are currently as far apart as possible (not sure how far) and tipped a bit forward (so as to minimize coupling to the stacks). There is a 75ohm quarter wave co-phased cable already in the truck (because a quarter wave of 75ohm coax will transform 50ohm at each antenna to 100ohm at the other end, and two 100ohm loads in parallel is once again 50ohm for the radio). My plan is to use a VNA and a separate half-wave cable (because half wave has an input impedance closest to the load connected to it), of course calibrated to the end anyhow, to individually assess the impedance at each antenna.
Here is my question: If I tune each antenna such that their feed points are 50ąj0 (or as close as possible), will the mutual coupling when they both see RF further alter the feed point impedance's or will they remain 50ohm even in the near field of the other antenna? If so, can anyone give me a hint as to how far the induced currents will shift the impedance away from 50ohm at the antennas? |
Dale
Senior Member
Username: Dale
Post Number: 2407
Registered: 12-2002
| | Posted on Saturday, June 05, 2021 - 9:53 am: |
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i didnt have a antenna anylizer way back when i done mine i just used a swr meter.. i used a singe 18 feet of rg58 coax to match each one in at time . then once were both done i hooked up the co phased 75 ohm coax.biggest thing nowdays are alot of these neweer semis are fiberglass not metal making it almost impossible to properly tune them..only option there is to get a firestick ngp or no groundplane .
dale/a.k.a.hotrod
cef426
cvc#64
454 [dx numbers]
38lsb
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Ke0koy
Junior Member
Username: Ke0koy
Post Number: 35
Registered: 12-2018
| | Posted on Monday, June 07, 2021 - 9:45 pm: |
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Thanks for the reply! What do you think about running a tuner in that case?
I've also been dreaming of ways to drive the stacks themselves. They are too short to do a j-pole thing with them, but perhaps a folded vertical by adding a spaced conductor parallel to the stack connected at the top and driven at the bottom. If I had a truck, I'd be trying everything. Unfortunately, I only have a short time to make something work on that rig. |
Dale
Senior Member
Username: Dale
Post Number: 2410
Registered: 12-2002
| | Posted on Tuesday, June 08, 2021 - 9:17 pm: |
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im not sold on tuners ..id rather tune the antenna itself ..im sure some may disagree im fine with that
dale/a.k.a.hotrod
cef426
cvc#64
454 [dx numbers]
38lsb
|
Ke0koy
Junior Member
Username: Ke0koy
Post Number: 36
Registered: 12-2018
| | Posted on Thursday, June 10, 2021 - 10:22 pm: |
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Yea, I am not sold on them either (but I do believe they work). I do not fully understand tuners. I get the concept of conjugate matching (same resistance, opposite but equal reactance so they cancel) and have been using smith charts for a while, but the physics of the electron flow still confuse me. I've heard many ways of how tuners physically operate (not just on paper with math), and the theory I initially heard (and disagreed with) ended up kicking my @$$. Me, I'm a book nerd. One of those people that knows everything without first-hand experience. Needless to say, every one of my hobbies has humbled me a bit more than the last, a little bit of experience at a time.
Anyhow, someone told me that a tuner does nothing more than create a better reflection point to send the reflected signal from the mismatched antenna back towards the antenna repeatedly until the antenna absorbs it all. Right then, I was the book-worm equivalent of a triggered liberal! Ticked off, I connected a transistor in avalanche mode to a coax connected to a tuner (which then connected to a mismatched load) with the oscilloscope tee'd in. My idea was to shoot that idea down by recording the fact there was not reflections bouncing back and forth from a single pulse. I failed in making that video. Then, I thought, it must be that my little <1ns rise time device wasn't 50ohm. I did the same thing with the radio itself, and once again, recorded nothing in support of the idea reflections didn't happen between the tuner and the antenna. But, that still didn't explain why the coax did not get exponentially hotter... Still just as confused as before.
All confusion aside, however, there exists a bit of self-evident truth. 1) The radio sees a low swr so nothing can be getting back to the radio. 2) The tuner, nor the coax, seems to even get warm, so 3) however it works, the antenna must eat it up somehow because my heat-detecting skin (nor IR thermometer) can feel the tuner or coax getting warm (meaning the watts are being used elsewhere?!). I have come to terms with the notion tuners work (like gravity) because the math says it has to, but admit that the math, as simple as hyperbolic trigonometry is, does not quite explain it for me. |
