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Tlcarrig
New member
Username: Tlcarrig
Post Number: 1
Registered: 10-2006
| | Posted on Wednesday, October 11, 2006 - 11:48 pm: | 
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Sorry for another SWR question but I really need some useful advice here. This forum was recomended to me in 3 out of 5 replies to a post on a motor home forum so ya'll must be good. I am installing a new Uniden PC78LTW in a factory pre-wired motor home that is 6 years old. To make sure I could get a decent SWR before I started chopping on my stainless whip, I made a test whip ftom 8 guage copper. I used this size because it would stand up without drooping. I was able to get 1:1 on channels 1 and 40 at 31 inches from the tip to the set screw. I put in the stainless whip and started looking for the "sweet spot" by choping an inch at a time. The stainless whip is now 25 inches "short" and the SWR meter still hasn't moved from pegged.Now what? Did I miss the spot by removing too much each time? Do I keep on going shorter or do I get a new longer whip and start over. I know the set up can get 1:1 because I had it with the copper and it stays steady at 1:1 if I put the copper back in there. I can't use that permanently though because it won't stay standing up. There is no coax connector on the antenna end. It's factory sealed in to the load coil. Also they didn't leave enough slack in the coax to cut and install a connector to replace the antenna. Help!!! |
Tech237
Moderator
Username: Tech237
Post Number: 555
Registered: 4-2004
| | Posted on Thursday, October 12, 2006 - 9:57 am: |
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Try using a length of wire coat hanger cut to the same size as the copper and see if that has the same SWR as the copper rod did.
With the steel whip is the SWR the same at both ends of the band or is it higher at one than the other?
it sounds like you might not be getting a good connection between the main part of the antenna and the steel whip. Part of the problem could also be the differences in thickness of the copper rod and the steel whip. Wire thickness can have an affect on resonance of a tuned circuit - and that is exactly what an antenna is.
***Tech 833 smart alec remark*** "No more wire hangers!!" - Mommy Dearest. |
Hotwire
Senior Member
Username: Hotwire
Post Number: 1893
Registered: 1-2005
| | Posted on Thursday, October 12, 2006 - 2:06 pm: |
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Are you saying that the swr meter is pegged with the stainless whip? But with the copper rod you get 1.1 swr? IF so thats odd. Now you will not be able to use the same length steel whip as the copper rod because copper has a higher velocity factor than steel. If you can get a 2.0 swr with the SS whip then go with it. Yeah swr is important but not as critical as some think. Nobody on the recieving end can tell the difference between 1.1 and 2.0. Now when you use amps and gadgets then swr gets more important. Usually when a swr meter is pegged it means that you have a short or your WAY out of resonance.
Rarely a good antenna ever needs pruning.
Feed us some more info and we can help out better.
What brand antenna/whip
What type of mount? magnet, bumper, roof mount,clamp
Where on the vehicle is the antenna?
You may need to get a new antenna. Cutting whips is bad. |
Tlcarrig
New member
Username: Tlcarrig
Post Number: 2
Registered: 10-2006
| | Posted on Thursday, October 12, 2006 - 7:03 pm: |
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Will try to answer both of you here. Tech237 - Yes the SWR is the same on 1 and 40 as best I can tell because the meter pegs out. Weather permitting tomorrow, I will try to clean all connections and recheck everything. I understand the difference in transmissive properties of SS vs. copper. Hotwire - Yes, the meter pegs with the stainless whip and I get 1:1 with the copper. I know the final length will be different. I just got a new stainless 58" whip from Radio shack and went and dug out my three way meter to use instead of the radio meter. I'm going to try the coat hanger trick too. Don,t know the brand. It is a roof mount prewired by the MH company. It is on the forward area, centered side to side on my MH roof which is aluminum on an aluminum frame. The MH is 36' long and I would estimate 9-10 feet wide. I am going to get pictures tomorrow as there is a guy on my motor home forum wanting to see some things. Will get back to ya'll tomorrow if I can get this stuff done. |
Slugo4449
Intermediate Member
Username: Slugo4449
Post Number: 118
Registered: 7-2004
| | Posted on Thursday, October 12, 2006 - 9:04 pm: |
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I know that on my Hamstick antennas they recommend that you not insert the stinger all the way in but to raise it up just a hair so it doesn't bottom out into the rest of the antenna.
Do you have access to an antenna analyzer?
Marty
KG6QKJ |
Tlcarrig
New member
Username: Tlcarrig
Post Number: 3
Registered: 10-2006
| | Posted on Thursday, October 12, 2006 - 11:38 pm: |
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Hi slugo4449. I'll remember that if I get a chance to work on it tomorrow. The only meters I have is the one built into the radio and an old 3 in 1 from Radio Shack that measures transmitted power, modulation and SWR. I gonna plug it in and see how it compares to the meter in the radio. I'll post particulars tomorrow after dark if I get a chance to work on it. |
Therealporkchop
Advanced Member
Username: Therealporkchop
Post Number: 906
Registered: 11-2002
| | Posted on Friday, October 13, 2006 - 4:09 am: |
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If you're running a spring...it might be shorted out inside the coil. Check that. Check to see if it's shorting out to the body also...like it's touching somewhere against the actual vehicle. |
Hollowpoint445
Senior Member
Username: Hollowpoint445
Post Number: 1385
Registered: 6-2004
| | Posted on Friday, October 13, 2006 - 7:41 pm: |
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Because copper is a better conductor, the copper whip would be shorter than the stainless whip. If you started with the stainless whip at the same length as the copper wire, you started too short. For example, a 1/4 wave fiberglass whip with a copper conductor is only 96 inches, while a 1/4 wave stainless whip is 102 inches. Loaded antennas won't show 6 inches of difference, but I would imagine it'll be at least an inch or two - maybe more.
More thoughts....
Was it parked in the same location both times? VSWR can be influenced by location of large metal objects nearby - houses with aluminum siding for example. Measure the VSWR while it's parked in a wide open area. Drive it to a parking lot if you don't have a couple wavelenghts of space - about 60 feet.
I always trim whips 1/4 inch at a time, because 1/4 inch can make a fair difference in resonance. It's more critical with whips that don't have a tunable whip/tip, but it's still important when cutting the whip to make small changes so you don't overshoot the sweet spot. |
Tlcarrig
New member
Username: Tlcarrig
Post Number: 4
Registered: 10-2006
| | Posted on Friday, October 13, 2006 - 10:33 pm: |
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I hate to take up space but I have to respond to people trying to help. There is a good spring between the whip and load coil. Not shorted though or would not get a good SWR on the copper whip. You are correct about the SS having to be longer. I backed my MH away from the house as far as I could go get away from my power and phone line. Distance is now around 30 feet. I reinstalled my copper test whip and verified my previous readings. I then put a coat hanger as long as I could get streight. It was a couple of inches longer than the copper. Bingo, too short. I verified this by cutting an inch off and the SWR went even higher. I decided to bite the bullet and put in the new 58" whip. I whittled on it until it's now around 48". No change in SWR. Both meters I am using peg on 1 and 40. After making many, many trips up and down my 12' step ladder I decided to take a break and go to my local Ace hdw. I got a 36' piece of 1/8" piano wire. I have made three 1/2" cuts on it and have seen the SWR drop into almost a useable range. Having run out of daylight, I will continue tomorrow. If I get this piano wire to give me what I want, I'm going to use it instead of the SS whip unless anyone knows why I shouldn't. Thanks all. |
Hollowpoint445
Senior Member
Username: Hollowpoint445
Post Number: 1387
Registered: 6-2004
| | Posted on Saturday, October 14, 2006 - 9:44 am: |
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I'm guessing that 48 inches is probably still much too long because the copper wire was 31 inches when it showed good VSWR. I would trim the stainless whip to 40 inches and start taking off 1/2 inch at a time until you see it start to drop - making sure to recalibrate the meter each time. When you see a drop, I'd start taking off 1/4 inch increments until it looks good and you have enough room to adjust for best VSWR. Access to an antenna analyzer would really help to see where the resonance point is right now, and what happens with each 1/4 inch change.
I don't know anything about piano wire, but as long as it's stainless I don't see any reason not to use it in place of the whip, but I'd still use the stainless whip if I already paid for it.
You can trim the piano wire to get a general idea of how long the stainless whip should be. I'd still add another couple of inches before rough cutting the stainless whip just to be safe, and then trim 1/4 inch at a time until the VSWR is good. |
Tech237
Moderator
Username: Tech237
Post Number: 561
Registered: 4-2004
| | Posted on Saturday, October 14, 2006 - 7:14 pm: |
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It wont hurt to try Hollowpoints suggetion but if the worst comes to pass use the piano wire with no worries. I have in the past left the coat hanger wire in use when I have had similar problems. |
Tlcarrig
New member
Username: Tlcarrig
Post Number: 5
Registered: 10-2006
| | Posted on Sunday, October 15, 2006 - 10:02 pm: |
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I'm going to request this thread be closed and I thank all for your suggestions. I got the piano wire down to 1.2:1 on channel 1, 1.5:1 on channel 40 and 1.35 on channel 19. Its still just a tad long but this is good enough for now. I have a 300 mile trip coming up the end of next week so I'll try it out then. |
Tech237
Moderator
Username: Tech237
Post Number: 575
Registered: 4-2004
| | Posted on Wednesday, October 18, 2006 - 10:28 am: |
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TL those SWRs are fine and you really do not need to try to reduce them any further.
Like everyone ele on this Forum we are always glad to be able to help anyone that needs it. Jut remember collectively we would have over a 1000 years of experience. |
Bruce
Senior Member
Username: Bruce
Post Number: 4127
Registered: 9-2003
| | Posted on Wednesday, October 18, 2006 - 1:22 pm: |
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SWR 1:1 power ( forward ) 100%
1.5:1 ..................96%
2.0:1 ..................90%
3.0:1 ..................75%
Even at 3:1 3 out of 4 watts goes out ... |
Therealporkchop
Advanced Member
Username: Therealporkchop
Post Number: 922
Registered: 11-2002
| | Posted on Thursday, October 19, 2006 - 12:07 am: |
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My new guess would be capacitance between the antenna and the body of the vehicle. The other "whip" you are using might not conduct that same capacitance as the original whip. Worth looking into, could be your whole issue. Try moving the original setup to the top of the vehicle and see if you get the same results or try it on another vehicle. Then you can rule out whether or not you have anything wrong with your current stuff.
Chop |
