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Author Topic: Just a concern, but asking for advice on this anyhow.  (Read 3412 times)
AJ3O
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Posts: 124




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« on: March 07, 2011, 01:48:25 PM »

I have a CB radio in my work truck, along with my FT-857D. The CB antenna is in the center of the roof on a mag-mount and I have a dual band 2m/70cm mag-mount fairly close but not touching. This set up has been working great. Last week I went to a Ham-fest and got a great deal on a 10m hamstick and mount along with the cable. I mounted it on the drivers side with a fender/hood lip mount. It has been one heck of a receive antenna (I was listening in on every band this weekend during the contest and the only problem I had was trying to find just one person to listen to out of all the talking....) and I am just waiting to try it out when I get the time to jump into a net or just chat with somebody.

Here is the question. If I am on 10m-20m(or any HF for that matter), and I have to answer someone on the CB for work, will I blow the receiver out on my FT-857D? It has NOT been modded and I plan for it to stay that way AND out of the repair shop. I know that the farther I am from the 11m frequencies, the safer I will be, but if it is better to shut off the FT-857D while transmitting on the CB, then so be it. They can wait a few minutes while I talk a truck in.

So I will just wait for the responses/flaming to begin.... hihi
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K0BG
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« Reply #1 on: March 07, 2011, 03:01:44 PM »

Not very likely, even if they were tuned to the same band. You should keep the antennas as far apart as you can, however.

I suspect you'd have (or will have) more interference problems due to common mode current because of the mag mounts.
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AJ3O
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« Reply #2 on: March 07, 2011, 03:58:41 PM »

The 11m antenna in the center of the roof is approximately four feet away from the fender mounted 10m antenna.

What if the 10m hamstick was tuned to monitor 11m channel 19, or any frequency close to it, and I was to talk on the CB, that would most likely do damage then. Correct? The CB and the Wilson trucker 5000 has outperformed many of the "Big-rig" show-offs pumping their systems up with "2 pills+" and dual antenna systems...... I can hear up to 15 miles and talk out to 10 miles confirmed all in the hilly Laurel Mountains of PA on a bad day. So that system works very well for what it is.

I only expanded the question for an example as the hamstick is really surprising me as a receive antenna for MANY of the bands. I was listening to a few rag chewing sessions on 20m, 40m, and a few qsl's on 60m. Soooooo, on the way back to my office, I tuned into ch. 19 to compare receive characteristics between the two antennas. It was like listening to a stereo CB! If I were to get absent minded, as I sometimes do, and forget to change bands or turn off the FT-857D, I would more than likely "fry" something then. Is this right Alan?
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K0BG
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« Reply #3 on: March 07, 2011, 04:16:02 PM »

When a otherwise resonant antenna, has the ability to receive on a wide bandwidth, it is indicative that the antenna, as a system, is lossy.
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AD5X
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« Reply #4 on: March 07, 2011, 04:16:17 PM »

I'm going to disagree a bit with Alan.  I start getting a little concerned when subjecting the receiver to a +10dBm signal.  This is 2Vpp, which is enough to forward bias diodes - so I figure this is a lot of signal to be putting into a receiver.  And +10dBm is 83dB above S9.  Maybe I'm too conservative here, but this is just my opinion.  So lets run through some numbers.

Your CB puts out 5-watts, or +37dBm.  So you need 27dB of isolation to drop this to +10dBm.  Now I don't know what the isolation is between two antennas that have very similar resonant frequencies and are just a few feet apart, but I would worry about this.

Even worse would be transmitting on your FT-857D.  This is +50dBm (at 100 watts).  So to get down to +10dBm into the CB receiver, you'd need 40dB of isolation.

It would be interesting to measure the antenna-to-antenna isolation.  Connect a 50 ohm resistor from the receive antenna to ground and connect a simple RF detector across it (see www.qsl.net/n9zia/wireless/pics/rfprobe.gif).  Measure the detected voltage and convert this to dBm.  Knowing your transmit power in dBm (+50dBm for 100 watt transmitter, +37dBm for a 5-watt CB rig), simply subtract the detected/calculated receive power from the transmit power (both in dBm) and you'll know your antenna-to-antenna isolation.

And incidentally, you can damage a receiver even when it is turned off.

Phil - AD5X
« Last Edit: March 07, 2011, 04:25:06 PM by AD5X » Logged
AJ3O
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« Reply #5 on: March 07, 2011, 07:19:53 PM »

Whooops, I haven't talked on the CB while the FT-857D was on 10m, buuuut, I just happened to notice the RF meter on the CB go up and down as I keyed the 10m during tune up. It was OFF. I guess that would explain that happening. Yes, I tuned it on a clear frequency and my external meter showed a flat 1:1 swr, and I lowered the power down to 5 watts out during the tuning.

So, I guess what I am asking now is: How do I protect my equipment from each other?  Undecided



By the way, thanks for the replies from all.
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AD5X
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« Reply #6 on: March 07, 2011, 08:22:19 PM »

You might be able to use decoupling stubs.  Here's an excellant book that shows how its done for contesters who are operating multiple radios on the same or different bands, with antennas in close proximity: www.inrad.net/product.php?productid=248&cat=148&page=1

Phil - AD5X
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WX7G
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« Reply #7 on: March 08, 2011, 08:36:36 AM »

The 11 meter transmitter will induce a couple of watts into the 10 meter receiver. I'm betting it will damage the FT-857. Give it a try and tell us what happens.
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AJ3O
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« Reply #8 on: March 09, 2011, 06:32:46 AM »

The 11 meter transmitter will induce a couple of watts into the 10 meter receiver. I'm betting it will damage the FT-857. Give it a try and tell us what happens.

Is this with the FT-857 tuned to 10 meters, or on any band, or even when turned off?

I have only used the CB four or five times while listening to the FT-857, but the FT-857 was on the 2 meter band and I also talked on the FT-857 on 2 meters while the CB was listening. Neither seemed to have been affected, but when I noticed the CB's RF needle moving with 5 watts tuning the FT-857 and the 10 meter antenna last week, this is what made me question this.

But at the same time, there are MANY over the road drivers (Long Haul Truckers, small freight, dump trucks, etc.) that ARE hams and run many setups similar to what I have. Yet, I have not seen this really explained as to how their equipment/setups have been or how well the different systems interacted with each other. There are also various emergency vehicles with more than one radio on different bands, they seem to play well with each other.

What about all these truckers pulled over in the rest stops parked not more than 5 feet away from the next truck, and one guy decides fires up his "AMPED" radio. Does the guy next to him watch his radio go up in smoke?

In all honesty, if the CB goes, I would be slightly upset, but if I ruin the FT-857, well, let's just say that I would probably have a meltdown.........

I really do appreciate the responses. Just to clear the air, I am a HAM, not a CB'er. While the CB is used for work between our trucks and deliveries to sites, my main interest is the amateur hobby.
Your opinions and recommendations are very valuable. Thanks again.
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WX7G
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« Reply #9 on: March 09, 2011, 06:57:48 AM »

I would think it would be as harmful with the FT-857 ON or OFF.

However, having said that, I park my 40 meter mobile with an FT-857 55' from a vertical transmitting antenna running 1200 watts. A NEC simuation shows that with 5.7 amps on the TX antenna 1.5 amps is induced in the RX antenna. The FT-857 was OFF. There is no damage.

I ran a NEC simulation of your situation and the 5 watt CB can induce 0.2 amps of antenna current in the 10 meter Hamstick. From my actual experience this should not harm the FT-857 if it is OFF. I would avoid doing this though unless you're ready to send the FT-857 in for repair.
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KD4LLA
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« Reply #10 on: March 11, 2011, 10:00:39 AM »

I used to have a Dodge mini-pickup with a HTX-100 10m, Alinco 2m, and a R/S CB.  The 10m and 2m antenna were located at the front corners of the truck bed, and were 4 feet apart.  I used a coupler on the built in AM/FM antenna for the CB.  Never had any interaction or problems.  But then again I did not try keying all at once...

Mike
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AJ3O
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Posts: 124




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« Reply #11 on: March 11, 2011, 01:56:52 PM »

I wouldn't key up both at the same time. I asked because there are lulls in the work while waiting for the dump trucks to return on site at which time I have to let them know where to get loaded, where to dump, or where to wait, etc. During those brief periods, I would like to chat it up with some of the locals on 2m or hit some of the repeaters and chat with people near my home on 2m or 70cm. I also drive up to 80 miles one way for work on the highway and the CB is very nice to monitor for "road conditions" even though I frequently shut it down due to obnoxious, tired, and angry drivers. I didn't want to be listening on one and talking on the other, then get VERY surprised by a noxious cloud of smoke from either....

So far I have been OK but I am not looking for another investment. But one buddy of mine mentioned that some public service vehicles have multiple radios and antennas with the antennas all within one to two feet from each other on the trunk deck or roof at the rear. How can their radios not fry each other out? Especially the state troopers that have two or three antennas that look to all be cut for the same frequency.

My head is starting to hurt.  Embarrassed
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