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Author Topic: Strange dual-band mobile problem  (Read 9691 times)
W7AYU
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« on: September 09, 2010, 12:58:59 PM »

I recently completed my third mobile installation. I have a Yaesu FT-7800R (dual-band 2m/70cm) mounted inside with a roof mounted dual-band high-gain antenna installed on an NMO mount. The NMO mount is installed right into the roof, and wired down the driver's side support beam to the radio. Across the entire 2m band, the highest SWR is 1.5:1. But, when I switch to 70cm, the SWR jumps to >=6:1 anywhere on the band. This same setup worked fine in my other truck and in my car. This leads me to believe that the problem might be related to feed line lengths. Could the feed line length impact the SWR on 70cm that badly? Would shortening the cable help?
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K0BG
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« Reply #1 on: September 09, 2010, 01:54:31 PM »

You didn't mention the antenna brand or model, and that would help.

As for the coax. The best length is the one that stretches from the antenna to the back of the transceiver. Any amount of extra length just adds loss especially at UHF. You should check the coax connections obviously.

This all assumes the antenna is tuned correctly. If the antenna is a Larsen NMO2/70, no tuning is necessary. Some of the imports require tuning, some do not.

Lastly, what are you using to measure the SWR, and where in the line is it?
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W7AYU
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« Reply #2 on: September 09, 2010, 02:41:46 PM »

You didn't mention the antenna brand or model, and that would help.
It's a Diamond SG7500NMO. It's factory pre-tuned and has performed flawlessly in the past. Incidentally, I did try swapping it out with a 1/4 wave 2m whip, which also showed the same results.

As for the coax. The best length is the one that stretches from the antenna to the back of the transceiver. Any amount of extra length just adds loss especially at UHF. You should check the coax connections obviously.
Point taken. I will check the connector, but if the connector is bad, wouldn't it be bad on both 2m and 70cm?

Lastly, what are you using to measure the SWR, and where in the line is it?
I'm using a small, portable VHF/UHF SWR meter attached at the transceiver.
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K0BG
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« Reply #3 on: September 09, 2010, 02:59:52 PM »

First, make sure the set screws in whip connectors are tight. The top one isn't a coil by the way, it's just a connector. You can usually remove it, and get by on 2 meters, but not 70 cm.

SWR is a complex function. It is easy to present a case where the SWR is high on one band, and low on another due to a bad coax connection, especially at the antenna. This includes the return (ground as it were).

The correct place to measure the SWR is as close to the antenna as you can. Since the input impedance is a complex one, any coax cable you have between the antenna, and the SWR meter, will effect the readout. This is especially true when using inexpensive SWR meters.
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KE3WD
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« Reply #4 on: September 09, 2010, 05:19:59 PM »

What coax did you use? 
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W7AYU
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« Reply #5 on: September 09, 2010, 09:21:14 PM »

What coax did you use? 

The coax is RG-58U.
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KI4SDY
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« Reply #6 on: September 09, 2010, 10:05:31 PM »

Here is my guess, based on past experience. I believe there is something wrong with your feedline and or connectors. The reason the SWR is higher on UHF is because feedline loss will be greater at the higher frequency. The length of the coax should make no difference if the antenna system is properly installed, but I personally would have at least used 8X, since UHF was in the mix.  Smiley

As was suggested, check the SWR of the antenna with a known good short jumper near the antenna base. If the SWR is the same there, then it is an antenna problem. If the high SWR on UHF disappears, then it is a feedline problem.   Wink

In most cases, when new cable and connectors are used in a feedline any problems are traced back to the connectors. If you installed the connectors on the cable, that is the first place to check!  Shocked

If you can't find the problem, get a local Elmer that is a mobile antenna guru to help rather than depending on blind suggestions from eHam.net "experts."  Cool That includes mine, only I am not an "expert!"  Lips sealed  

I would have suggested just slapping a MFJ 1729 power gain magnetic antenna on the roof for $39.99. The coax and connector is already installed. Think of the time and money you would have saved, plus you would have been up and running in five minutes with a great antenna!  Grin

Unfortunately, you already drilled a hole in your roof. Oh well, there are always rubber plugs.  Cry    

« Last Edit: September 10, 2010, 05:57:53 AM by Guy "Vern" Wells » Logged
K5LXP
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« Reply #7 on: September 10, 2010, 06:46:00 AM »

I agree with KI4SDY about a problem with feedline or connectors.  It may have been a perfect line kit when you put it in, but "stuff" happens- the line can get pinched or cut on trim or a sharp flange, a screw could go through it, a sharp bend kinks it or something could short out the underside of the antenna contact after everything is tightened down.

A tool that I use is an antennna contact (NMO, or whatever) to PL-259 adapter, so I can connect a dummy load and a wattmeter right at the antenna contact end.  If you put X watts into the radio end, you should see pretty much all of it coming out of the other.  With a dummy load connected in place of the antenna, you can also verify your SWR at the radio end as a double check.

RG-58 vs 8X won't make any difference.  For a typical 12 foot long line kit there's less than a half dB of difference between the two and I wouldn't use foam dielectric coax (like 8X) in a mobile installation for durability reasons. 


Mark K5LXP
Albuquerque, NM

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KI4SDY
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« Reply #8 on: September 10, 2010, 07:09:46 AM »

I have been using 8x in mobile installations for over a decade in the Florida heat with absolutely no durability problems and excellent performance! However, if any coax is installed improperly, such as pinching, cutting, piercing, or abrasion, then you are going to have issues. If you buy the coax in a box it usually tells you that in the instructions, but it should be common knowledge by most experienced hams. Wink
« Last Edit: September 10, 2010, 07:20:32 PM by Guy "Vern" Wells » Logged
K5LXP
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« Reply #9 on: September 10, 2010, 07:34:55 AM »

In my experience it requires more care in routing as the foam dielectric crushes more easily under heat/pressure, and center conductor migration from tight bend radius.  It's also a bit bigger, which isn't as convenient to route sometimes.  If 8X was a compelling choice, the OEM's would supply line kits with it.  Makes you stock a different set of connectors too.  For a 12' or shorter run I don't see any loss advantage. 


Mark K5LXP
Albuquerque, NM
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KI4SDY
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« Reply #10 on: September 10, 2010, 07:21:45 PM »

I guess someone needs to work on their cable installation skills!  Grin
« Last Edit: September 10, 2010, 09:10:12 PM by Guy "Vern" Wells » Logged
K5LXP
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« Reply #11 on: September 11, 2010, 08:30:13 AM »

Go ahead and start a new thread, and post your specific questions.  There are lots of guys here that will be able to help you with that.

Mark K5LXP
Albuquerque, NM
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KI4SDY
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« Reply #12 on: September 11, 2010, 08:35:30 AM »

I am not having any problems installing 8X in mobile installations, you are! Unfortunately, you won't read directions or seek any help.  Roll Eyes
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K3GM
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« Reply #13 on: September 11, 2010, 09:23:16 AM »

.........If you buy the coax in a box it usually tells you that in the instructions, but it should be common knowledge by most experienced hams. Wink
Coax in a box you say? Are you serious?  Sounds much more convenient than those unruly 1000' spools I have to deal with.

I have been using 8x in mobile installations for over a decade........
It says here that KI4SDY was issued 11/16/2006........
« Last Edit: September 11, 2010, 09:25:32 AM by Tom Hybiske » Logged
K0BG
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« Reply #14 on: September 11, 2010, 03:28:26 PM »

I use Belden 9258, which is about as good an RG8X as you can buy. It does have it faults. As Mark says, it will migrate, and you have to be very careful to keep the bend radius at least 3 inches. If it sits exposed to 100°+ temps, the jacket gets rather brittle after about 12 months. There are better choices, but they aren't cheap!
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