RF in the shack

[C2]

I decided to pull out the old ham gear and give it a go.

I'm using an Icom IC-710 rig on 80 - 10m on a random wire antenna with an MFJ tuner (the lesser 300W one). The rig is powered from a 25A switching supply. I ran a braid strap from the chassis of the power supply and radio. The power supply is DC grounded via its AC supply from a 3-wire cord.

The feed/antenna is about 20 feet of RG59U coax with a bannana plug soldered to the center conductor. The bannanna plug is stuck into the random wire port on my MFJ tuner. The coax braid is not connected to anything. This coax basically runs straight back to the shack wall and outside through a hole and is then soldered to the end of a bare, copper clad wire 132 feet long that is streatched between two trees with a piece of nylon rope at each end.

Since the coax shield is not used, I didn't care about it touching the shack wall and it just hangs from the hole in the wall to where it is soldered to the antenna, about 10 feet up at that point. The other end of the antenna is about 30 feet up.

Now, I tune up 80 meters while sending CQ. I'm only running about 10W and I get the reflect down to 0 as indicated on the cross needle meter on the tuner.

All seemed good, but I noticed that I feld some burning on my hand where it was lightly touching my radio as I was tuning the antenna capacitance knob on the tuner. I also caught a glimpse of my analog multi-meter, set to AC volts with the leads just sitting on the bench next to the radio, and it registered up to maybe 3 Volts. Also, my spectrum analyzer sitting on the bench was centered on the band with a 2-meter rubber duck antenna connected and it was showing -15 dBm of signal.

Now, I was looking at an online RF safety calculator and it is indicating that even at 100W into a dipole, you only need to be about 2 feet away, but it just seems wrong to see my multimeter swinging around.

I also noted that the radiated signal was much less (about -40 dBm) outside where the coax is soldered to the antenna. (not that I realize that the coax is also part of the antenna here, I'm just mentioning it because it's there).

 

[Sonwatcher]

I had the same thing with an offcenter dipole ( windom). I made a coax balun and it fixed it.

[SIZE=-1]The required length of the plastic pipe depends on the diameter and length of the coax used and the diameter of the pipe. For RG-213/U coax, about one foot of 5 inch size pipe is needed for a 1.8MHz to 30MHz balun. For 3.5MHz to 30MHz coverage, about 18 to 21 feet of coax is needed. This length of coax is also adequate for most applications on 1.8MHz.

18 to 21 feet should cover all of 160 through 10 meters.
The number of turns is not critical because the inductance depends more on the length of the wire (coax) than on the number of turns, which will vary depending on the diameter of the plastic pipe that is used.

The coax is single-layer close-wound on the plastic pipe.

The first and last turns of the coax are secured to the plastic pipe with nylon cable ties passed through small holes drilled in the plastic pipe.

The coil winding must not be placed against a conductor.
The name of this simple but effective device is a choke-balun.

NOTE: Some people build choke-baluns, without a plastic coil-form, by scramble-winding the coax into a coil and taping it together. The problem with scramble-winding is that the first and last turns of the coax may touch each other. This creates two complications. The distributed-capacitance of the balun is increased and the RF-lossy vinyl jacket of the coax is subjected to a high RF-voltage. The single-layer winding on the plastic coil-form construction method solves these problems since it divides the RF-voltage and capacitance evenly across each turn of the balun"....AG6K
[/SIZE]

http://www.klimaco.com/HAMRADIOPAGES/hf_baluns.htm

 

[Moleculo]

I'm assuming you grounded the Tuner also, correct?

I'm not sure I like the idea of just using the center conductor on the coax and leaving the braid like that. Try soldering the braid and center conductor on both sides together. I doubt that has anything to do with the RFI...

 

[Moleculo]

You know, after thinking about it some more...

Soldering the shield and center conductor togther will just make that all one radiating part of the antenna (of course). Rather than doing that, it might be better to use a regular piece of coax to feed the longwire with (PL-259 on one side, attached to tuner), grounding the sheild outside the shack where you attach the rest of the wire. The problem with bringing long wires into the shack is that unless the stray RF finds a path to ground, you're going to have it everywhere. Feeding it with coax also gives you the ability to try a 1:1 current balun or a choke as Sonwatcher desribed. That can't hurt, either.

 

[N9RZD]

I decided to pull out the old ham gear and give it a go.

I'm using an Icom IC-710 rig on 80 - 10m on a random wire antenna with an MFJ tuner (the lesser 300W one). The rig is powered from a 25A switching supply. I ran a braid strap from the chassis of the power supply and radio. The power supply is DC grounded via its AC supply from a 3-wire cord.

The feed/antenna is about 20 feet of RG59U coax with a bannana plug soldered to the center conductor. The bannanna plug is stuck into the random wire port on my MFJ tuner. The coax braid is not connected to anything. This coax basically runs straight back to the shack wall and outside through a hole and is then soldered to the end of a bare, copper clad wire 132 feet long that is streatched between two trees with a piece of nylon rope at each end.

Since the coax shield is not used, I didn't care about it touching the shack wall and it just hangs from the hole in the wall to where it is soldered to the antenna, about 10 feet up at that point. The other end of the antenna is about 30 feet up.

Now, I tune up 80 meters while sending CQ. I'm only running about 10W and I get the reflect down to 0 as indicated on the cross needle meter on the tuner.

All seemed good, but I noticed that I feld some burning on my hand where it was lightly touching my radio as I was tuning the antenna capacitance knob on the tuner. I also caught a glimpse of my analog multi-meter, set to AC volts with the leads just sitting on the bench next to the radio, and it registered up to maybe 3 Volts. Also, my spectrum analyzer sitting on the bench was centered on the band with a 2-meter rubber duck antenna connected and it was showing -15 dBm of signal.

Now, I was looking at an online RF safety calculator and it is indicating that even at 100W into a dipole, you only need to be about 2 feet away, but it just seems wrong to see my multimeter swinging around.

I also noted that the radiated signal was much less (about -40 dBm) outside where the coax is soldered to the antenna. (not that I realize that the coax is also part of the antenna here, I'm just mentioning it because it's there).

try grounding the shield of the feed line to your shack ground ,
balun=good

 

[W5LZ]

Basically, what you have for an antenna is a random length wire from the tuner to where ever it ends up. The coax braid isn't shielding anything. From the sound of it, and if I'm understanding you correctly, you also do not have an RF ground. The AC ground is fine for safety's sake but that random length antenna needs an RF ground / counterpoise. The total antenna length is that 132 feet plus the length of the RG-59. (Or depending on the frequency, get ready for your mustache to catch on fire from the stray RF in the shack .)
The simplest thing to do would be to run a wire ground about the same length as your antenna. Get it outside however you can and just throw it out on the grass. That's the antenna ground/counterpoise, and is just to see what happens. Naturally it means your tuner settings will change, so re-tune and give it a try. It should at the very least, reduce the RF in the shack, and it will make a difference in what you can hear/how you'll be heard. If things are working satisfactorily, you can do something about that 'ground' wire/counterpoise later. As in get it under the grass so the lawn mower doesn't get clogged up or you trip on it, sort of. You can also think of that wire as a radial, which work better than ground rods except for 'safety grounds'. The usual 'ROT' (rule of thumb) is as much wire in the ground as in the air. Or even more depending on the conductivity of the soil. ('Best' is copper plating the yard!)
Not being familiar with your tuner, and if you keep the present antenna, does the tuner have a connection for a 'long wire' or 'random wire' or unbalanced antenna? That's the one to use. (Typically threaded post(s), not an SO-239.) That's one of the nice things about using a dipole for each band, they don't require an RF ground.
- 'Doc

 

[Beetle]

Doc beat me to it. Your antenna is an end-fed wire with no counterpoise or RF ground - at least you didn't mention having a counterpoise. You absolutely need such an RF ground if you expect the antenna system to work well at all. Without any counterpoise, the RF is going to try its best to find something, starting at the feedpoint -- which in your case is where the banana plug attaches to your tuner. That whole thing is the antenna.

Since the RF can't find anything, it's coupling to whatever metallic surfaces it can find in the area, which includes all of your gear, your mike, etc.

Sounds like you have a bit of work to do.

 

[dudmuck]

<snip> copper clad wire 132 feet long that is streatched between two trees with a piece of nylon rope at each end.

If you have that much space, you can use the tried & tested carolina windom.
The version that works on 80 meters is 133 feet long.
This antenna runs much of the 80, 40, 20, 10 meters without a tuner. And anything in between with a tuner.

 

[C2]

Yeah, it's basically just an end-fed random wire antenna.

The back of the tuner:

The wire is just stuck into the Wire port and I'm not using a jumper to the balanced line ports (the balanced line ports connect to a balun internally).

I used the ground wingnut to connect 100 feet of wire and ran that outside along the ground, but it basically runs close to where the antenna is (the lot is 30' x 135' and the shack has to be in the back corner). And, adding this additional wire seemed to make no difference so I took it off. I suspected this was because the coax jumper from the tuner to the radio makes the ground, which in turn connects to the mains earth ground, and that feeds the shack through an underground conduit.

One more item, the bench is an 8-foot steel frame bench with a power strip bolted on, and is supplied through a 50 foot extension cord to the sub-panel (where the conduit comes in).

 

[949Jake]

This sounds like the kind of antenna I would feed with ladder line, if I had any.

 

[Captain Kilowatt]

I run an end fed longwire here. The coax feeder shield is grounded at the shack end and open at the feedpoint. No balun,what's the sense? It's not a balanced antenna and neither is the feeder. I have very minimal RF in the shack and only when on 160m with 100 watts and all it does is make my computer speakers HUMMMMMMMMMM a little.