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Lmr-400 for CB use

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swen_out_west

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Lmr-400 for CB use with IMax 2000

Alright time to pick the minds of Hams that read the CB section or any CB'r for that matter.

So based upon significantly less DB loss I figured I could use 50' of LMR-400 for my IMax-2000 and it turns out there must be a mismatch somewhere in this configuration, since my normal 1.2 VSWR (SWR) has now shot to 2.1. I checked all connections and it didn't help. Replaced it with the old rg-8x and it's back to 1.2.

So this has me perplexed. I just picked up 200 feet of Heliax that should arrive next week at a good price of .75 cents a foot. Am I going to have the same problem with the Heliax. Or is there a chance that the cable is bad and all I really need to do is replace the connectors. FYI, the two cables (LMR-400 vs, RG-8x are exactly the same length).

So I might have overreacted but I bought a MFJ 915 Balun for use with that Heliax when it gets here. Either way I want to try it out anyways.

Is there something I am missing. Does anybody run LMR-400 for CB? Or just keep taking it down and turn the tuning rings then put it back up. I didn't have to mess with the rings when I use Rg-8x and does anyone know what way I turn them. I've looked online and I get conflicting answers.

The most common answer is that if your SWR is higher on 40 than on 1, turn the rings up.
if higher on 1 than 40, turn the rings down. But is that really going to fix the problem, right now 1 is 2.1 and 40 is 1,9.

I think I am just going to put the Rg-8x back on and deal with it until the Heliax gets here and then figure it out.
 
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swen_out_west

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Alright I was just messing around and hooked up my antenna switch using a RG-8x jumper since currently the LMR-400 is on it.

the VSWR dropped to 1,9 on ch 1 and 1.5 on ch 40, does this not prove that a mismatch has occurred. If that's the case won't that MFJ 915 Balun rectify this.

Either way I don't really want to use it like this and will be dropping the antenna again tmrw afternoon when my help comes back and just put the RG-8x back on.
 
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buddrousa

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Back in the day anytime you had or made a change to coax you had to tune your antenna. Retune your antenna first low band has always been a little on the picky side.
 

jonwienke

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There's nothing wrong with using LMR400 for CB, as long as the cable is in good condition, all connectors are properly attached, etc. IMO you've got something else going on that just the cable. Changing cable type shouldn't be a large enough impedance difference to make that much of a change in SWR.

I suspect you don't have the ground plane kit for your IMAX-2000, and your coax shield is acting as your RF ground. If that is the case, changing feedline anywhere (on the tower or in the house) will have random effects on your SWR reading. If you don't have the ground plane kit, get it. It would be ridiculous to run heliax but not have the ground plane--the ground plane will do far more for antenna performance than the difference between LMR400 and heliax, and it will cut down on RF radiating from your coax shield. It's sort of the RF equivalent of adding a spoiler when the engine is misfiring on two cylinders.

Also, unless you're measuring SWR with the meter attached directly to the antenna, switching to a less lossy coax will raise SWR readings simply because less of the signal reflected back from the antenna is being attenuated. It's not that the antenna match got worse, it's that you're getting a more accurate measurement of how bad it really is.
 

prcguy

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Two things potentially going on here. Whatever the match is on the antenna, it will be apparently reduced when you have loss between the SWR meter including coax loss. If you have a 2:1 match at the antenna it could look like 1.5:1 or less depending on loss. If you upgrade the coax to something with less loss it will show a match closer to reality, meaning your LMR400 could be showing you a more realistic match on your antenna.

Another thing is the IMAX200 and similar antennas are poorly decoupled from the feed line and the feed line will radiate some. The particular length of coax you use along with the mast length can change the match slightly. It could be you found a lucky bad coax length that affects the SWR in a bad way.

You should measure the match right at the antenna with the meter on the antenna for a sanity check. If its 2:1 then the LMR400 is showing you what's actually there. If its a great match at the antenna then add some odd length jumpers and see if the match stays constant or wanders all over the place with different lengths of coax.
prcguy
 

PACNWDude

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LMR-400 and RG-213 has worked for my CB use for years....just weatherize outside connections with vulcanizing tape if you want it to last.
 

jim202

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The above information is good.

A simple change of the coax should not effect the SWR of the antenna. If it does, then maybe there was a high loss in the old cable that was hiding the reflected power from being seen at the radio. A simple check can be made by putting a watt meter at the far end of the coax with a dummy load. Measure the TX power at the far end. Then pull the watt meter and put the dummy load on the coax. Move the watt meter to the radio end of the cable and measure the TX output power again. The difference is the cable loss. In 50 feet of cable, I would not expect to see much if any difference.

I would do testing on a few things at this point.

If you have an ohm meter, disconnect the LMR400 from both the antenna and the radio. Check to make sure you don't have a short in the cable. Put one lead on the shell of the coax connector. Place the second lead on the center pin of the coax connector. You should not see anything but infinity. Then put a 50 Ohm termination on the far end of the coax cable. Do the test again and you should see about 50 Ohms. You can double check your reading by doing the same test right on the dummy load RF connector.

If you have a dummy load you know is good, put it on the end of the LMR400 and connect it back up to the radio. Do the normal transmit check at channel one and again at channel forty. You should see a very low SWR on both channels.

Connect the coax back up to the antenna. I forgot the readings you supplied already. But if the reading is lower at the high channel, and higher at the lower channel, this indicates that the antenna is probably too short. On the other hand, if the reading is higher at the lower channel and lower at the high channel, this indicates your antenna probably is not long enough.

The instructions indicate that there is a screw adjustment at the base to lengthen or shorten the entire antenna by rotating it. I couldn't figure out how to secure the screw turning to keep it from moving with the limited information provided. Maybe if I had the antenna in front of me it would be more obvious.

There is also a ground plane kit that is available for that antenna. I didn't do any research on just what the effects of the ground plane hit have. I will say that the length of the ground plane elements will effect the SWR if you do have that kit installed.

This information is not much to go on, but maybe it will give you some ideas.

Jim
 

swen_out_west

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yeah, I was tired posting this last night pretty much what I thought but was just trying to verify it in my head.. I agree that it's either a bad cable or the Rg-8x was giving me faulty readings, gonna do some checks this afternoon on the antenna end of that cable.

I do have the ground plane attached. Since my reading is only .2 off between channels I really don't think it needs much adjustment,
 

jonwienke

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But if the reading is lower at the high channel, and higher at the lower channel, this indicates that the antenna is probably too short. On the other hand, if the reading is higher at the lower channel and lower at the high channel, this indicates your antenna probably is not long enough.
But if the reading is lower at the low channel, and higher at the high channel, this indicates that the antenna is too long. On the other hand, if the reading is higher at the low channel and lower at the high channel, this indicates your antenna is too short.

Fixed it for you.
 

SpugEddy

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Just an FYI.
I recently made the change from RG8 to LMR400
I can tell you this: Don't expect to see major changes.
You may see a "slight" drop in SWR but don't expect to
see major change in Rx or Tx. I wanna say my SWR
dropped from about 2:1 to about 1.8:1
Aside from the small SWR drop I see no difference in
my Reception nor Transmit.
 

AC9BX

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If everything is working properly you should not expect to see significant change in SWR by changing the feedline of equal impedance and in the same position in this case. (yes changing position makes a difference)

However, if your cables do not have exactly the same impedance there will a difference. And if the antenna is not exactly 50 Ohms and the radio is (which is also not likely the case) there will be differences if cable impedances are not the same.

50 feet of LMR400 at 27 MHz will yield about .3dB loss.
50 feet of Belden 9258 (an RG8X size cable) will yield about .8dB loss. We're talking about .5dB improvement. It's so small it's hardly worth noting. This assumes a perfect antenna and healthy cable. What is the RG8X?

SWR is a ratio. It's the amount of reflected power compared to forward output power. With a meter at the radio for example it reads the output power there. But this signal goes to the antenna, some comes back, with loss along the cable thus the measured reflected power at the measurement point is wrong, less than what is actually being reflected by the antenna. This is always the case. The concern is how much reflected power the radio can tolerate. (the lossy cable is better for the radio) The .5dB we're looking at shouldn't change that very much either.

Most likely the reason you see apparent better SWR with the old cable versus new is because it's much more lossy. Coax gets lossy with age. Sometimes people look at their SWR and see their system getting better over time. Of course it isn't, your coax is getting worse.

With exceptions, typically antennas with tuning purposely to this point, best SWR does not indicate best radiating efficiency. Don't be overly concerned with SWR. It's good to monitor as an indicator that something has changed.
 

swen_out_west

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I have tuned the antenna closer but even perfectly tuned, the problem was still there. Haven't taken my checks at the end of the line yet, already summer in the desert so I am not outside in the hot sun.

Pretty sure at this point the 8x was attenuating my SWR reading. This was a real problem, with my galaxy I could hear the Standing Wave discharging after you transmitted.

I also have proven that it's a bad length. Adding length does change the reading and I almost have it down to 1.5 now..

I'm not going to mess with it at this point. I like the LMR-400, my receive is much better than with the RG-8x. (Being in the middle of nowhere, .5 db loss of receive is a huge difference, My buddy is going to borrow me a tuner at this point which might compensate for this problem and then we can adjust the length accordingly when I install the Heliax.

I always rolled my eyes when CB'rs talked about multiples of 18 feet. Based on velocity factor this is a fallacy. However, there is something to be said about the right length once you factor in the velocity factor. I just happened to hit a bad spot on my wavelength.
 
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jonwienke

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I always rolled my eyes when CB'rs talked about multiples of 18 feet. Based on velocity factor this is a fallacy. However, there is something to be said about the right length once you factor in the velocity factor. I just happened to hit a bad spot on my wavelength.
No, you will still have the same problem when you install the Heliax. You're radiating from the coax shield because of poor decoupling between antenna and coax, and using "magic lengths" of coax merely masks the problem. Fix the problem at the antenna with a common-mode choke. It's much easier to make one from LMR400 than from heliax because LMR400 has a smaller bend radius.
 

swen_out_west

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So I wasn't overreacting and was on the right track when I bought that MFJ-915 when this all started. Just checked my tracking. It should be here by noon on Monday.

Just need to find what works the best. A well made product from MFJ or a homemade choke.
 
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swen_out_west

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So I made an 'Ugly' Choke for the end. Had my girlfriends son check the mail as he came on the property before we lifted the antenna and he came with a MFJ-915.

Good lord the skip is busy tonight but with running that LMR-400 I have never had a receive on my IMAX 2000 like I have tonight. The SWR is 1.1 on both 1 and 40. I'm having a huge problem in the fact that I have way too much in my ear.

Next weekend I'm gonna get that Y-Quad up just so I can focus in one direction.
 
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toastycookies

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I just picked up 200 feet of Heliax that should arrive next week at a good price of .75 cents a foot.
LDF-150?

Just asking because I've been getting a bunch of this too for cheap and I don't understand why people would ever go back to braided coax except for the availability of connectors.

Hardline all the way for me :D
 

jim202

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LDF-150?

Just asking because I've been getting a bunch of this too for cheap and I don't understand why people would ever go back to braided coax except for the availability of connectors.

Hardline all the way for me :D
Availability of the connectors is not the problem, it's the sticker price of the connectors.
 

mmckenna

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Availability of the connectors is not the problem, it's the sticker price of the connectors.
E-bay and similar sources are a good place to pick up cheap Heliax connectors. Also, in many cases, the connectors can be reused. I usually save the old ones and use them in non-critical applications or give them to local amateurs.
 
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