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Message |
    
Keithinatlanta
Advanced Member
Username: Keithinatlanta
Post Number: 778
Registered: 3-2003
| | Posted on Saturday, July 29, 2006 - 7:22 am: | 
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Ok guys, "HOW" do you adjust if you have high swr when using the whip antenna? Just start cutting on the 18' of coax? If so, how much do you cut off at a time? Thanks.
Keith |
Hollowpoint445
Senior Member
Username: Hollowpoint445
Post Number: 1321
Registered: 6-2004
| | Posted on Saturday, July 29, 2006 - 10:50 am: |
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You shouldn't cut coax to match an antenna. What kind of problem are you having with what kind of antenna? |
Tech237
Moderator
Username: Tech237
Post Number: 466
Registered: 4-2004
| | Posted on Saturday, July 29, 2006 - 11:18 am: |
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Keith,
The only thing that cutting the coax will acheive is to allow your meter to see a different SWR. I have sent an article into this forum that describes what varying the length of coax will do.
In short an electrical hlafwave length of coax will show you a more accurate rading at teh shack/radio end than a random length and that is all it will do. To tune an antenna you have to adjust the length of the antenna itself.
Firstly tho I'd be inclined to recheck all connections, especially the ground, make sure you have connections where you should and that you do not have any shorts.
Like Hollowpoint asked what type of antenna?? |
Tech808
Moderator
Username: Tech808
Post Number: 10665
Registered: 8-2002
| | Posted on Saturday, July 29, 2006 - 11:19 am: |
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Keithinatlanta,
What kind of Whip Antenna is it?
If it is a 102" Stainless Steel Whip are you using the SPRING?
The MORE information you can give us the better chance we have of helping you.
Lon
Tech808
CEF808
N9CEF
CVC#2 |
Hotwire
Senior Member
Username: Hotwire
Post Number: 1601
Registered: 1-2005
| | Posted on Saturday, July 29, 2006 - 12:01 pm: |
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Keith,Are you using the 102 inch stainless steel whip? For the cb band you will want to get as close to 108 inches as possible. When I installed mine I used the ball mount which is 2 inches in diameter and a 4 inch spring. My swr is flat! Also VERY important to get as much of the antenna above all the vehicle as you can. You do not want the bottom of the whip very close to the side of the car! good luck
Hey if you like have a look at members pictures at my 102 inch install! It may help. I also wrote how I did it. It may be of some very good ideas for ya! |
Dale
Advanced Member
Username: Dale
Post Number: 586
Registered: 12-2002
| | Posted on Saturday, July 29, 2006 - 12:44 pm: |
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ok dumb question.what is a electrical half wavelenth of coax?is it 18ft. or is it every 9ft |
Hollowpoint445
Senior Member
Username: Hollowpoint445
Post Number: 1322
Registered: 6-2004
| | Posted on Saturday, July 29, 2006 - 1:52 pm: |
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Dale - it depends on the velocity factor of the coax and the frequency you will use most. |
Tech237
Moderator
Username: Tech237
Post Number: 469
Registered: 4-2004
| | Posted on Saturday, July 29, 2006 - 5:38 pm: |
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Neither Dale. For most 50 ohm coax (RG-58 etc) it will be around 66% of a physical halfwave length. So that makes it approximately - 11.88ft or 11ft 10 - 1/2 inches - including the plugs.
But watch out - a couple of rare 50 ohm coaxs have a Velocity factor of 75%. |
Dale
Advanced Member
Username: Dale
Post Number: 589
Registered: 12-2002
| | Posted on Sunday, July 30, 2006 - 12:45 pm: |
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ok.i wonder why most coax jumpers are 9,18 ft then especially in mobile .mostr of my instals 18ft was way too much |
Tech237
Moderator
Username: Tech237
Post Number: 472
Registered: 4-2004
| | Posted on Monday, July 31, 2006 - 10:18 am: |
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Convenient length for the manufacturer, who probably does not really understand the effect coax lengths can have. |
Keithinatlanta
Advanced Member
Username: Keithinatlanta
Post Number: 780
Registered: 3-2003
| | Posted on Tuesday, August 01, 2006 - 7:21 am: |
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Francis Fiberglass whip 67"'s in length with new stainless steel spring mounted on old style but brand new "chain" bumper mount. ( any of you old enough to remember those?  ) Clearance over height of camper shell is 30 "'s. Antenna is mounted on right side in rear. Fiberglass camper shell, Z 71 Ext. Cab, short bed, (not shorty bed) Chevrolet.
I used this antenna back in late 80's on a Ford ext. cab. Had no swr problems at all.
The only radios I use are 40 channel CB radios. I do NOT use linears either. So just a straight forty channel CB, changing out radios sometimes, but usually the President McKinley with AM/SSB or the Uniden PC 78 LTW that has the 7 weather bands.
PLEASE NOTE:
I do NOT have SWR problems now. I have not even bought the coax yet. My question is this. If I use 18' of coax, hopefully I will not have any problem. But if I do, HOW do you adjust for swr?
One thing to keep in mind is this. Ext cab with short bed is a pretty long truck, especially when threading the coax under the truck for clearance and safety, and then under carpet, etc. inside. By the way, radio is mounted inside the dashboard. Looks great in there too. Maybe Lon or someone can tell me how to e mail a picture in the future so yall can see.
Hopefully I have given more info now. Thanks to everyone for their input. Keep it simple please. I do not need the technical info on coax. Just how to adust for swr if I should run into a problem.
By the way. When I ran this antenna back in 80's with a President Grant 40 channel AM/SSB, I talked all over the place. And NO peak or tune either!
Keith in Atlanta
CEF 150 |
Hotwire
Senior Member
Username: Hotwire
Post Number: 1610
Registered: 1-2005
| | Posted on Tuesday, August 01, 2006 - 9:25 am: |
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KiethinAtlanta, Francis pretunes their antennas in the factory. The spring is gonna change your swr. Try it with and without the spring and if possible move the mount around on the bumper. Some may say to trim the antenna but I say NO! It will ruin its resonance to cb! Just go with the lowest SWR you can get and go with that. If the lowest you can get is a 2.0 that will work. Nobody will ever know the difference between a flat swr and a 2.0 swr on the recieving end. So many cbers pull their hair out with swr over such a small percentage of power. Just mount that thing and hollar for the Hotwire! good luck
P.S. Just my 2 cents. I have used Francis before and have worked many many states and Canada barefoot! They make very good barefoot antennas. Just do not seem to hold a lot of power. Still have a couple Wheeler Dealers and HotRods now! |
Vanillagorilla
Advanced Member
Username: Vanillagorilla
Post Number: 708
Registered: 4-2005
| | Posted on Tuesday, August 01, 2006 - 10:34 am: |
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I Say..put it together first. Pat says odd multiples and I think its was hollowpoint that advocates 1/2 wave multiples. Either version you do have to take into account the velocity factor.
I'd suggest taking the reading at the antenna feedpoint IF you think you have a problem.
If you see 1.2:1 and are ok with that then setup your SWR meter at the radio end to see if both readings jive.
IF the readings at the radio are way out from what you saw at the antenna end THEN you have a coax problem. If the coax is new and the reading is slightly different then you might want to trim coax as needed till you see the same SWR at both ends.
I myself cannot say whether you want 1/2 wave or odd multiples or whatever. All I can say is that according to theory the 27mhz wavelength repeats the wavelength every 1/2 wave which is about 17' IN FREE SPACE...velocity factor not included. This is MY understanding.
Your mission is to see that the coax ENDS at the end of a repeating cycle. Anything more (or less) may be returned to the radio and SEEN in the SWR meter as high SWR even though your antenna is already resonant.
It will depend on how fast the signal moves (VF) to determine where your theoretical 1/2 wave sits within your coax.
Whatever coax you chose..look up its rated volocity factor and apply it to your application.
Taking Tech237 example...for example
RG-58 coax @ 66% VF
17' (of RG-58..to start..1/2 wave) X 66% (Velocity factor of RG-58) = 11.22' overall length of coax giving you 1/2 wave within YOUR coax OR looked at the way you and your radio want...50 Ohm impedance.
See..its that simple 
I'll guess that 11.22' is not long enough for your application. Looking at a calculater you can now double that sum and use 22.44' with the same results.
34 (17x2) X 66% VF = 22.44' OR 11.22 X 2
Yep...44.88' works too. If you wanna put your whip on the rear bumper of a SCHOOL BUS!
Oh scrap all this. Put it together first! Theory Shmeory! Real world results is what counts!
Just don't go clipping your whip till your SURE all else is good....like your ground/counterpoise.
Oh lets not go there! |
Vanillagorilla
Advanced Member
Username: Vanillagorilla
Post Number: 709
Registered: 4-2005
| | Posted on Tuesday, August 01, 2006 - 10:55 am: |
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Btw...Keith. This is not for your application but more for your information.
IF you had a whip that NEEDED trimming:
If the SWR were higher on 40 than on 1 than your antenna is electronically "long" I'd start by trimming no more than 1/4"....because you can't put it back if too much is removed. Trim till you are equal between 1 and 40 frequently checking ch19 or 20. Make it flat across the band NOT being so concerned with a perfect 1:1 match. Anything below or around 1.5:1 is fine!
I DID do this for my own purpose..right or wrong.
I had a Shakespeare big stick BOTTOM half given to me a while back with a short. After repairing it I decided to set it up on the garage as a secondary low power/around town station in the back yard. Well I had no top section. I just kept farming through my Glass whips till I found one close enough in electronic length. Some where WAY to short and some were of course way too long and not deserving of the hacksaw. I found an old radio shack trucker "wound" topload that needed a tiny trim to get me resonant....yep..I checked at the feedpoint and radio. I used older unknown origin RG8X size coax that worked out to around 20' and I see 1.3:1 across the band...talks 15mi barefoot at 10' to feed point ...go figure...
Good luck!
Hank |
