| Author |
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vernonott
| | Posted on Tuesday, June 12, 2001 - 6:46 am: | 
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Good Morning 307 : How do you set SWR on 102 " whip?My friend has one that is 3:1 .We have checked mounting and ground ,looks okay. |
307 (307)
| | Posted on Tuesday, June 12, 2001 - 10:27 am: |
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You do not set the SWR on a 102" whip , you have other trouble. A 102" whip is a perfect match UNLESS you have a bad ground , bad coax , bad mount or bad PL-259's |
honky tonk man 593
| | Posted on Tuesday, June 12, 2001 - 4:42 pm: |
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even off my bumper with the antenna only 2 inches form the tail gate i only had a swr of 1.5 so i bet your ground is bad try changing the coax and if that doesnt work....try another meter youd be surprised how many times an old meter can be wrong. hey good luck
honky tonk man 593 outta new york!! |
155ALAMO
| | Posted on Thursday, August 09, 2001 - 7:39 pm: |
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I HAVE 1.0 SWR, MOUNTED RIGHT BEHIND THE CAB OF MY PICKUP, ABOUT A FOOT FROM THE CAB.
USE 17 FT OF COAX. THAT ALWAYS SEEMS TO WORK FOR ME. |
chab
| | Posted on Friday, August 10, 2001 - 12:42 pm: |
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1.0 is impossable, 1.1 is as good as it gets |
Skullman
| | Posted on Friday, August 10, 2001 - 7:05 pm: |
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Check the ground first. Make sure that little plastic washer in your mount isn't broke or cracked second. If all else fails trim the top of the anntenna 1/8 of an inch at a time untill the SWR's go down. |
vernonott
| | Posted on Saturday, August 11, 2001 - 6:27 pm: |
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Skullman : This is the first time I've ever heard of trimming anything off of a 102 " whip. |
FlaGator66
| | Posted on Sunday, August 12, 2001 - 2:15 pm: |
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When I first installed My 102" whip I had SWR readings that told me the antenna was too short I put a 14" extension rod between the Mount and the Stud Now I have SWR 1:2 on Channel 1 and 40 and yes If your initial SWR readings say that your are too long You can trim the antenna or the coax to bring it down
FlaGator66 |
vernonott
| | Posted on Sunday, August 12, 2001 - 8:15 pm: |
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Look at me ,I have for over twenty years thought that exactly 102" length whip was the most efficient and trouble free antenna ever built for 11 meter. |
Skullman
| | Posted on Sunday, August 12, 2001 - 8:17 pm: |
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Vern, it's not a real good idea to trim one, but it is a last resort to fixing high SWR's. |
vernonott
| | Posted on Monday, August 13, 2001 - 3:51 pm: |
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I have lowered the swr on many 102's by adding jumpers.Just last week a fellow came to me with a high swr on his 102".I checked the ohms through his coax and antenna and got a reading of 39 ohms.I added a 3' jumper and the ohms rose to 51 ohms with a swr of 1.2.If I had needed to go the other way I would have taken three feet of coax off.I will continue to play with the coax but you don't have to worry about me trimming a 102" antenna.102" is 102" is 102". |
Skullman
| | Posted on Monday, August 13, 2001 - 6:26 pm: |
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Oh, yeah, vern, it's a good idea to play with the coax lengths first. What I meant was sometimes I run into people who want thier radio in a particular place in their vehicle and have the shortest coax they can stand to reach that location. In that situation trimming coax is not an option and cutting the antenna to acheive a match is the only option. Beleive me I don't do it either except in a necessary situation. P.S. These are also people who insit on running a cellular look a like ant because they don't want a big bulky antenna. Go figure. |
bruce
| | Posted on Tuesday, August 14, 2001 - 12:22 pm: |
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IF your wip is cut to 27 and IF your coax is realy 50 ohms then there is no way the coax will change swr. What you are doing is moving the meter down the standing wave and when you hit the zero voltage point your meter beleves you have a good match check ARRL antenna books for real matching solutions before covering your floor with coax cuttings. |
307 (307)
| | Posted on Tuesday, August 14, 2001 - 12:38 pm: |
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Thats the problem , Most off the shelf coax is not at 50 ohms , it may be close but you are asking for a lot to be right at 50 ohms.. |
707
| | Posted on Tuesday, August 14, 2001 - 6:37 pm: |
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Skull..just a sidebar note:
a buddy of mine insisted on one of those cellulike cb antennas on his vintage El Camino. It actually worked ok. I think the record was one evening when I was able talk to him all the way out to about 30 miles on AM, me on a 40' A99 with an old barefoot 77-882c, him barefoot running a 2950(original model). Kept losing him on the hills, but I was pretty impressed. I could never get one of those to work more than a couple of miles. |
Skullman
| | Posted on Wednesday, August 15, 2001 - 5:50 pm: |
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Well, 707, you can credit that to two things, a good radio on his part (I love 2950's) and a great antenna on your part. Consider him very lucky, I still have no luck with those things, but I can't say that satisfactory results can't be had. 73
CEO of DEADMAN INC. |
Snakedoctor
| | Posted on Tuesday, August 21, 2001 - 4:17 am: |
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Hey guys....back years ago a lot of the whips sold for CB were 108". I still have one that dates back to the early 60's. Later the 102" whips became more popular. Never knew why they changed, but my old antenna came from ALLIED ELECTRONICS when I bought my old white face Johnson. When I bought the whip it came with papers telling how to trim it for good SWR's but I never needed to because it was very good as it was. |
vernonott
| | Posted on Thursday, August 23, 2001 - 6:47 am: |
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Snakedoctor :If my memory serves me correctly people used to buy the 108" whip and add a six inch spring under it making it 114" , so they started making it 102" and with the spring you have 108". |
boobietrap
| | Posted on Tuesday, September 04, 2001 - 5:23 pm: |
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Now that you all have gotten your 102 in. whips whiped in shape, try taking 102 inches of 10 gage wire, attaching a terminal to one end with a diameter large enough for the threaded end of the whip, and winding the wire in a tight coil up the antenna, then wrap it with good electrical tape. I tested it and it seemed to lower the radiation pattern and I get 2 s-units more on the recieving radio. Don't forget to recheck the swr and trim the coil if needed. It did raise the swr on mine from 1.2 to 1, to 1.5 to one but that is negligible. |
super
| | Posted on Thursday, September 06, 2001 - 3:26 am: |
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hey boobietrap are you talking about the run of the mill hardware store 10ga stranded insulated wire? |
boobietrap
| | Posted on Saturday, September 08, 2001 - 7:59 am: |
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Yes super, just regular 10 gage wire. Honest it does make a difference, pick a spot a few miles away from a friend in a fixed location, get a good meter reading, add the modification with the car in the exact position and check it again, you will be suprised. |
Joe Juneau
| | Posted on Friday, September 21, 2001 - 7:56 am: |
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How do you check the ohms on coax and antena? I have a 102" steel whip, when I check the ohms with a digital multimeter I get 0L on the display. When I disconnect the coax and check it I get the same reading.When I short one end to shield and check it I get 0.0 ohms. Am I missing something? Thanks for any info. Joe |
Tech181
| | Posted on Friday, September 21, 2001 - 1:01 pm: |
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Joe,
You have to disconnect both ends of the coax or else the meter will show shorted. The 0.0 reading when you connect center to ground is waht you want. Does the antenna perform properly?
Steve
Tech181
Tech181@copperelectronics.com |
HAM CBer
| | Posted on Saturday, September 22, 2001 - 1:28 am: |
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Joe,
You check the impedance (ohms) of the antenna system with either an Operating Impedance bridge (the right way), or an MFJ antenna analyzer (the cheap but still O.K. way). An ohm meter only measures the system at DC potential. You need to measure the system at operating frequency for an meaningful reading.
You not only worry about ohms impedance, but ohms reactance too. Adding or taking away coax will NOT change the impedance of the antenna system (remember, it is all a SYSTEM), but will slightly change the reactance. If your antenna system is slightly capacitive, adding more coax can introduce just enough inductance to get the reactance closer to j0.
To your radio, coax looks like a bunch of inductors in series with capacitors to ground. There is no magic length of coax, but for some dumb reason, 17-18 feet or RG-58 seems to make capacitive antennas happier. |
Joe Juneau aka.. GI JOE
| | Posted on Saturday, September 22, 2001 - 1:10 pm: |
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Steve,
I assume my antenna is working properly, I am having swr problems, and that's what I am trying to figure out. I have a Uniden PC76XL with a D104-M6B mic, 18' of RG8X and a 102" whip. My radio has been modified with a hi/lo switch. On lo I key 2 swr and it swings to 5 swr when I audi in the mic and on hi I key 2.5 and it stays the same when I audi and the ant lite comes on every time I talk in hi or lo. I know it is normal in most cases for the swrs to increase and the ant lite to come on when a radio is modified but I would like to get my swrs lower if possible and some of the messages in the forum talks about measuring the impedance of the coax and antena to see how close to 50 ohms it really is. I would like to be able to measure it to see if I could lower the swrs or if I would be wasting time and money cutting coax and maybe buying another antenna.
Thanks, JOE |
Tech181
| | Posted on Sunday, September 23, 2001 - 12:15 am: |
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GI Joe,
Where is your antenna mounted?
Is it grounded well?
Is the antenna free to radiate with no metallic obstructions?
Did you check the mount?
Is the mount electrically isolated from the center lug?
Is the coax in good shape?
Are the connectors in good shape?
Also, are you using an external SWR meter or relying on the one in the radio?
Steve
Tech181
Tech181@copperelectronics.com |
G I JOE
| | Posted on Sunday, September 23, 2001 - 12:15 pm: |
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Steve
Tech181,
My antenna is mounted 2' 6" from the rear windshield on an aluminum toolbox that is bolted to the floor of the bed with four bolts with star washers, according to my meter the ground is excelent.The ony obstruction or reflection I may have is the bottom 2'5" of the antenna is bellow the top of the cab. The insulator disk, coax, and connectors are new and everything checks out ok with a multimeter. I have even tried another piece of coax. I have a Radio Shak swr meter, but it reads the same as a Dosey that a friend has. I may be wasting your time, with the mods done to my radio, this may be normal.
Thanks Again,
G I JOE |
HAM CBer
| | Posted on Sunday, September 23, 2001 - 1:43 pm: |
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That's not normal for a radio with 'mods' to have a higher SWR than a stock radio. In fact, your SWR should not change at all whether you are running 1 watt or 1,000 watts. A 'modified' radio putting out 6 more watts than before should make no difference at all.... Unless...........
Have someone check the output of your radio on a spectrum analyzer. If the 'technician' that tuned up your radio did not have it hooked up to a spectrum analyzer while doing it, and tuned it for maximum 'smoke' without regard to minimum spurious emissions, you may have a case of a radio putting out lots of spurious emissions. You would have no way of knowing without a spectrum analyzer. A watt meter will show all the output of a radio regardless of what frequency it is, and the radio would still 'sound' fine on the air. If your radio is putting out spurs all over the place including out of band, that would obviously make your SWR look bad since the spurs are on frequencies your antenna is not tuned to. Not to mention the potential for interference to out of band radio services and all the power you lose that is not being concentrated on the frequency you want.
I run into this all the time. It is more common than you think.
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G I Joe
| | Posted on Tuesday, September 25, 2001 - 5:57 pm: |
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HAM CBer,
Thanks for the info, I'm gonna try to find someone with a spectrum analyzer. I did hook up another radio and cecked the swrs and they were the same as what my radio shows on hi side. I was not able to compare the lo side, for the other radio is not modified with a hi/lo switch. Having similar results with an unmodified radio, is there still a chance mine may have spurious emissions?
Also, I have some RG 8/U coax with a solid center conductor. It was used for a cctv security system at a casino, what I have is what was left over. Could it be used for a base?
THANKS,
G I Joe |
HAM CBer
| | Posted on Tuesday, September 25, 2001 - 9:17 pm: |
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Are you SURE that coax is RG8? Most CATV and CCTV systems use 75 ohm cable, not 50 ohm like you're supposed to for your CB.
Just because the coax is the same size as RG8 does not make it RG8. RG11 is 75 ohm coax that looks EXACTLY like RG8 unless you read what was printed on the jacket.
Either way, used coax sucks. Unless it's Heliax, you might want to pass it up. |
highlander
| | Posted on Wednesday, September 26, 2001 - 7:53 am: |
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Sometimes 75 ohm coax is ok... have a 40 foot run of 75 ohm between my a-99 and radio---swr is 1.1 at 27 mhz, and curves to 1.3 at 26 and 28---perfect 11 meter curve! Sometimes the antenna feedpoint isn't 50 ohms, anyway. Why did I try this in the first place? Because I got 150 feet of the stuff for free! |
HAM CBer
| | Posted on Wednesday, September 26, 2001 - 2:08 pm: |
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That's not true. The ONLY time 75 coax is O.K. is in a co-phasing harness where the feedpoint is summed between the two separate runs. If your radio wants to see a 50 ohm load and you show it a different load, you will have reflected power (SWR).
If you have 75 ohm coax going to your A99 and your match is good, all that means is that you are LOSING a lot of signal between your rig and the antenna. Replacing that coax with good quality 50 ohm cable like Belden 9913 or Times LMR-400 would improve your TX and RX by about 4dB or more.
That's simple physics. I don't have the desire to go into the particulars here. |
G I Joe
| | Posted on Wednesday, September 26, 2001 - 6:42 pm: |
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The coax is new, and it is RG8. My only concern was that the center conductor is solid instead of stranded. |
HAM CBer
| | Posted on Wednesday, September 26, 2001 - 10:13 pm: |
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Does it have a foil shield in addition to the strands? What does the jacket say exactly?
If you tell me what the jacket has printed on it, I'll try to look up that cable and see if there is anything goofy with it (IE: velocity factor, capacitance variance, etc.)
Hey Forum Master: Is the password option going to be enabled very soon?? I'm sticking my neck out here.... |
Hamcber
| | Posted on Thursday, September 27, 2001 - 1:55 am: |
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Testing the new features. Please disregard. |
444
| | Posted on Saturday, September 29, 2001 - 8:53 pm: |
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a 102inch whip is NOT a "perfect match" on 11m... 1/4wave is 108inches, the 102inch whip needs the spring.
If you are getting a 3:1 SWR, you need to look elsewhere, shorted coax, open coax, whip too close to the car/truck.
My calculator says that your SWR using a 102in whip at 27.2MHz, the SWR should be pretty close to 1.9:1
Speed of light (rounded to 300e6) divided by freq in Hertz
300,000,000/f (hz) = wavelength in meters
Divide by 4 = 1/4wave in METERS |
highlander
| | Posted on Sunday, September 30, 2001 - 12:18 am: |
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Ham CBer, I'm quite sure you are mistaken on this one. Go ahead and go into it here, let me know how a 1.1 to 1 swr isnt really a 1.1 to 1 swr just because I am using 75 ohm feedline. If I use 50 ohm feedline with one antenna system, and the meter shows 1.5 to 1, that is ok, right? So then I put a different antenna system together, using 75 ohm feedline, and the same swr meter, and it shows 1.1 to 1, you are telling me that that is a false reading? Try again. Get your nose back in that ARRL handbook and tell me about it. There is alot you are not taking into consideration. |
G I JOE
| | Posted on Sunday, September 30, 2001 - 3:39 pm: |
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HAM CBer,
The coax does have a foil shield under the braided shield,and it also has a clear plastic looking insulator. The outer jacket says...BELDEN 9913 50 OHM RG8/U TYPE COAX HS...
By the way, a friend of mine has a 102" whip with the bottom cut off and slid down inside of a 2' pole. I put his setup on my truck and adjusted the swr by sliding the whip up and down, and was able to get a 1.1:1 swr. I guess I just havent yet found the right length of coax for my rig. Thanks for all the help everyone, it was greatly appreciated.
G I JOE |
Hamcber
| | Posted on Sunday, September 30, 2001 - 1:53 pm: |
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444 is correct, a 102 inch whip is not a perfect match. Even a 102 inch whip with a six inch spring are not a perfect match. However, the impedance and reactance of a 108 inch whip should be pretty close to perfect on 27 MHz.
highlander, You are misquoting me. If your meter shows a 1:1 (0%) SWR, that only means that your radio is not seeing as much reflected power as a 1.5:1 (4%) SWR.
Your 75 ohm coaxial cable is losing enough of your transmitter power that it is not getting as much power to the antenna feedpoint and the small percentage of reflected power back again to your radio. The correct way to measure an antenna's SWR is at the feedpoint, not at the radio. Let's say you use 50 ohm 95% shield coax and you have 4 watts at the antenna base. Then, you substitute similarly shielded, similar velocity factor and capacitance 75 ohm coax. With the 75 ohm cable you will not see the same 4 watts at the antenna base. Same thing works in reverse (reflected).
So, if your antenna has a 1.5:1 SWR at the antenna base with 50 ohm cable, it still has that same SWR with 75 ohm cable too. This is all assuming that your standing wave reference is to 50 ohms (like CB's are designed for). Since the SWR is still 1.5:1 at the antenna base, and your feedpoint SWR dropped to 1:1, your 75 ohm cable has not magically 'fixed' the feedpoint mismatch and allowed full transmitter (and receiver) power energy transfer. You have simply lost enough power that your full 4 watts has not made it to the antenna and your 4% reflected power has not made it back to your radio. That is called LOSS.
Oh, by the way, I do not get my information from the ARRL handbook. I am a professional engineer with 20 years of experience in design of high and low power transmitter and antenna systems. I have designed megawatt SW antennas down to milliwatt 1.9 gig systems. I have designed and built AM and FM antennas and directional arrays. I have designed and built AM broadcast arrays that have commonpoint impedances from 16 to 62 ohms (that's what they were licensed for). I know a thing or two about antennas from professional training and experience.
Quote: "Go ahead and go into it here"
I only said "I don't have the desire to go into the particulars here", sorry about that. If you would like to know more info about this, please swing by my shop and I will most gladly discuss in person. Email and message boards are too time consuming and don't allow me to show you on a Smith Chart what I am talking about. I'm always happy to have visitors looking for professional information! The above info is all you get for now. |
Vernonott
| | Posted on Monday, October 01, 2001 - 8:44 am: |
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Good to have you back on the forum Hamcber. |
Hamcber
| | Posted on Monday, October 01, 2001 - 12:07 pm: |
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Thanks.
Actually, I was waiting for the password option to become available before getting back in to it. I am now beta testing the feature for the Forum Master.
The only thing that is different is that I apparently cannot be 'HAM CBer', that now I have to be 'Hamcber'. I don't know why, but I'm just happy that now some idiot can't pose as me and give horribly incorrect information to the unsuspecting again. Even though all the spelling and grammar errors should have been a clue..... It seems that our education system will pass students whom cannot spell or structure a sentence. Either that, or those whom wish to pose as me did not graduate.
If you click on my name, you will see my 'profile'. This is going to improve the board greatly once it is implemented for everyone.
HAM CBer..... eerrrrrrr..... Hamcber. |
Vernonott
| | Posted on Monday, October 01, 2001 - 8:26 pm: |
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I checked out your profile . Do you always smile that big?I am impressed with your antenna systems expertise.I too am testing out the system. |
G I JOE
| | Posted on Thursday, October 25, 2001 - 8:49 pm: |
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HAM CBer,
Have you had a chance to look up that coax I was asking about. I'm ready to put up a base and need to know if my coax is good enough. The entry in the forum started Sep 25 - Sep 30 in this section. THANKS,
G I JOE |
Marconi
| | Posted on Friday, October 26, 2001 - 7:57 am: |
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At the end of September GI Joe wrote:
"HAM CBer,
The coax does have a foil shield under the braided shield,and it also has a clear plastic looking insulator. The outer jacket says...BELDEN 9913 50 OHM RG8/U TYPE COAX HS...
By the way, a friend of mine has a 102" whip with the bottom cut off and slid down inside of a 2' pole. I put his setup on my truck and adjusted the swr by sliding the whip up and down, and was able to get a 1.1:1 swr. I guess I just havent yet found the right length of coax for my rig. Thanks for all the help everyone, it was greatly appreciated.
G I JOE"
Joe, the Belden should be air space insulated (ASPE) with a small coil of polyethylene between the #10 solid copper center conductor and the foil/copper shield.
You describe the following, "...clear plastic looking insulator." Is this solid are or you talking about the outer covering of the coax?
Just as a caution, this Belden 9913 is fine, but if it is ASPE then it does not handle bending very well. It is also prone to damage by water
as it acts like a suction, allowing water to get into the coax if not well protected at the exposed ends.
Did you use this same coax on your buddies system? His idea sounds like a good solution to make the whip adjustable. A 102" whip should be short for 11 meters. As noted above, I would avoid triming this antenna, they were intended to be used with a 6" spring.
For testing your antenna, you could easily do the same or similar at the tip of the antenna. If it is to short, then just add an 8" piece of wire holding it with a small hose clamp at the top. Trim this and see if you can get the whip resonant where you work your rig the most. If successful, then you could consider this additional length as a starting point at the bottom for a permanent type adjustable mount like your buddy.
BTW, I think you can rely on the advise from HamCBER. To this point, I have read nothing that he has said to be out of line eventhough he has a cartoonish looking face.
Marconi |
G I JOE
| | Posted on Friday, October 26, 2001 - 1:04 pm: |
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Marconi,
The outer jacket is black, the insulator I'm talking about is between the foil/braid shield and the center conductor, it looks and feels solid to me.
No, I didn't use this coax on my buddies system, we both use mini 8 in our mobiles and that was a problem I was having with swr in my mobile and just borrowed his antenna setup just to see if I was having radio problems or not. This coax I'm asking about is for my base that I'm going to set up soon.
Thanks,
G I JOE |
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