| Author |
Message |
    
Chris142
Member
Username: Chris142
Post Number: 96
Registered: 7-2002
| | Posted on Friday, May 28, 2010 - 10:48 pm: | 
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I have a 150w linear under my passenger seat of my jeep.
I bought a 5ft K40 antenna. It's very stiff so it wont bang against my roof and drive me banannas.
Now when I key the mic the Linear wont unkey. It keeps transmitting and it's transmit light stays on.
My match is almost too good to be true with this antenna. Match is 1.2 on all 40, dude at the truckstop CB shop was suprized too.
I went back to the flexable k40 antenna that bangs on my roof and the linear works normal now. Match is 1.3 on 1 and 1.6 on 40 so it's good too.
How can the longer antenna have an effect on the linear? Weird RF output or something?
When the long antenna is on and the linear is "stuck" transmitting I can unplug the coax and it will stop transmitting and the light will go out. Does not matter if I unplug the input or the output. |
Tech833
Intermediate Member
Username: Tech833
Post Number: 187
Registered: 12-2001
| | Posted on Saturday, May 29, 2010 - 12:52 pm: |
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The problem is RF feedback into the amplifier. The longer antenna converts more of your wattage into useable electromagnetic transmitted RF field, and that field puts enough field into your cables to keep enough RF flowing into the keying circuit. After you unkey your radio, the amp is going into oscillation and self-generating.
Since you are running 150 watts, I have to assume this is an amateur (ham) installation we are talking about. The first step is to make sure your equipment is well grounded. The chassis of the radio and amp should be grounded directly to the metal bodywork. Second, you need to install double shielded cables. Your typical RG-58 or even RG-8 won't work. The CA-400 cable from Copper is recommended. The shielding will help reduce the electrical field induced into the equipment from the antenna.
It is also worth noting that this is a STRONG field we are talking about. Make sure you don't have any body parts too close to that antenna, or don't transmit very long.
I have always been a fan of K40 antennas, they do work very well.
Your radio 'Mythbuster' since 1998
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Tech237
Moderator
Username: Tech237
Post Number: 1345
Registered: 4-2004
| | Posted on Saturday, May 29, 2010 - 10:49 am: |
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You are getting RF back into the system with the 5ft K40 and it is keeping the linear stuck in transmit mode. Does the radio itself stayed keyed or does it drop back to receive??
If the radio stays keyed the RF is getting into the radio, but if it unkeys, then the RF is getting into just the amp.
1.2:1 across the band on the 5ft, and the changing SWR on the other antenna makes me suspect a bad antenna.
Tech237
N7AUS
God only made some many perfect head, on the rest he put hair.
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Tech833
Intermediate Member
Username: Tech833
Post Number: 188
Registered: 12-2001
| | Posted on Saturday, May 29, 2010 - 1:07 pm: |
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"1.2:1 across the band on the 5ft, and the changing SWR on the other antenna makes me suspect a bad antenna."
Or, it could be that the new antenna has a better bandwidth than the skinny antenna. I found that out when doing the "Big Coil Antennas Exposed" article (out soon!). The skinny fiberglass antennas had such a high Q, the bandwidth was sometimes less than 1% of frequency. The large diameter fiberglass antennas had 3x the bandwidth of the skinny ones.
Your radio 'Mythbuster' since 1998
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Chris142
Member
Username: Chris142
Post Number: 97
Registered: 7-2002
| | Posted on Saturday, May 29, 2010 - 8:26 pm: |
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The radio recieves or tries to recieve when the linear is transmitting. It's a Galaxy 959 and the Linear is an old Triad.
Different coax eh? |
Chris142
Member
Username: Chris142
Post Number: 98
Registered: 7-2002
| | Posted on Saturday, May 29, 2010 - 8:28 pm: |
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"It is also worth noting that this is a STRONG field we are talking about. Make sure you don't have any body parts too close to that antenna, or don't transmit very long. "
What can happen? The jeep does not have a metal top. Am I cooking myself? |
Tech833
Moderator
Username: Tech833
Post Number: 1823
Registered: 8-2002
| | Posted on Saturday, May 29, 2010 - 8:43 pm: |
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For the sake of simplicity; Yes, you are.
According to my calculations, assuming 450 watts PEP (a 150 watt carrier will give you this on modulation peaks), you are exposing yourself to RF power levels above the safe limit at 5 feet away from your 5 foot tall antenna.
You can dowload a free MPE calculator here- http://www.qsl.net/w0jec/mpe5.htm and run the calculations yourself. Although this free calculator isn't as sophisticated as the ones I use, it will still give you an idea.
If you ran 5 watts carrier, you would be plenty safe at 5 feet.
Your radio 'Mythbuster' since 1998
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Fieldrat
Junior Member
Username: Fieldrat
Post Number: 19
Registered: 5-2011
| | Posted on Saturday, July 09, 2011 - 10:54 pm: |
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I am having a problem with a 102 whip, the swr are 1.4 on 1 and 20 and 1.15 on 40, but on 40 the radio meter shows it swinging backward, now it did not do this with the little wil magnet mount, it is a new cobra 29 bluetooth, and if u use the swr meter on the radio it will not cal. on 40 is it my ant. or the radio. |
Tech833
Moderator
Username: Tech833
Post Number: 2002
Registered: 8-2002
| | Posted on Sunday, July 10, 2011 - 2:56 pm: |
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Hmmm...
Assuming the whip is just a tad short (as it appears from your SWR curve), that means it is slightly capacitive at the feedpoint. If the radio is not happy driving this capacitive load, it would need to be retuned. Component drift over time, or most likely, a backyard "golden screwdriver" tech caused this.
The radio should be able to handle a slightly capacitive load no problem.
Other solution- add length to the antenna and make it slightly inductive. Most transmitters are happier into an inductive load.
Your radio 'Mythbuster' since 1998
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Dale
Senior Member
Username: Dale
Post Number: 1736
Registered: 12-2002
| | Posted on Monday, July 11, 2011 - 7:26 pm: |
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do u have a spring or riser on the 102?
cause i agree with tech 833 your whip
appears to be reading too sdhort.the lil wil
didnt do this probaly cause swr was better.
my guess.if ya have a spring or riser on it
already then ad a washer or two till 1-40
read closer to the same
dale/a.k.a.hotrod
cef426
cvc#64
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Fieldrat
Junior Member
Username: Fieldrat
Post Number: 20
Registered: 5-2011
| | Posted on Monday, July 11, 2011 - 10:15 pm: |
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Thanks alot guys, but now it is working fine, it is deadkey 3 watts and swinging about 15, and yes i do agree it needs to be length to get the swr even across the chanels, the radio is stock except for a swing mod to get the deadkey down so i could run it easy with my rm kl300 kicker, but it did not get it to the 1 watt it clamed it would, maybe kens electronic sent me the wrong parts, i just do not understand why it is working fine now, i have another 29 ltd classic that you have to give it a love tap evey now and then because it will quit rx and tx, bump the case and it takes back off, not been very happy with cobra since uniden quit building there parts, and now after this, don't know if its the radio or ant. I will get a spring or maybe just a riser, and keep a eye on it for a while, again thanks alot. |
