Longwire Antenna Q's

[nocomradio]

In the last year or so, I've gotten back into SWLing having been away since I was a kid. Now I have the advantage of a couple more bucks and better equipment, but I have a couple questions:

I'm doing most of my listening from a Grundig G3 Globe Traveler. It is an OK radio and does pretty good on the built in antenna. I decided to put up a longwire. It is approximately 220 feet long and wired into the house with a 75 ohm cable and plugged into the side of the radio at the external antenna port.

It helps to pull in some things, but honestly it seems to just make more noise and that in turn makes for harder listening. I have little in the way of interference in my immediate area at night as I am very rural and have no CF's or computers running at that hour.

Is there a standard length for a longwire?

Is it just a cheap radio I am dealing with?

Should I try to fix the length of the longwire to the approximate middle of the frequencies I listen to most?

Should it be 1/4, 1/2, or 1 full wavelength?

Any input would be greatly appreciated!

 

[nocomradio]

Forgot to add:

The longwire is a steel wire that is bare and suspended at one end about 8 feet off the ground and at the other approximately 15 feet high. It is pulled fairly taut.

 

[K6JHU]

For listening the antenna length does not make an awful lot of difference. On broadcast frequencies wavelength is about 300 meters so a quarter wave would be about 250 feet (all approximate).

You probably lose more energy with the impedance mismatch due to the conversion from the straight wire to 75 ohm coax. And is there a ground on the shield of the coax?

What makes nervous (and I don't have the manual) is that generally external AM antenna inputs are not 75 ohms. Typically just two screw terminals (antenna and ground),

 

[nocomradio]

The antenna lead is a standard 75 ohm antenna lead like that used for TV antenna. The center lead is attached to the long wire, and the ground shield is attached to the 75 ohm connector outer nut. The outer shield is essentially doing nothing as it isn't connected on the long wire end.

The radio has an 1/8" phone plug connection for the external antenna, and I am using a 1/8" to 75 ohm adapter, but I doubt it has a balun or any type of matching circuitry.

The radio manual isn't specific at all about antenna types and really all it says is that there is a jack for connecting an external antenna. I assume (probably shouldn't) that the tip of the 1/8" plug is the antenna lead and the sleeve is the ground.

Would grounding the coax help maybe?

 

[KeithE4]

Forgot to add:

The longwire is a steel wire that is bare and suspended at one end about 8 feet off the ground and at the other approximately 15 feet high. It is pulled fairly taut.

Use copper or aluminum, not steel, for antennas. Steel wire is lossy.

You must also match the antenna to the feedline, and the feedline to the radio. 75 ohm coax isn't good unless the antenna is cut properly for the desired band and odd harmonics thereof (which is why antennas cut for the 7 MHz ham band will also work on 21 MHz). If you must use coax, use good RG-6 TV cable instead of cheap CB-style RG-59. Less loss above 20 MHz (below that, it won't make much difference).

For receiving, bring the wire directly to the radio with an L- or pi-network tuner inline, or feed the antenna with 300 or 450 ohm twinlead or open-wire line and use a balanced tuner. I prefer the latter since it can be matched over a wide range of frequencies and even ultra-cheap TV twinlead of decades ago has less loss at HF than even the best coax, if properly matched at the radio.

Even a single 10-365 pf variable capacitor from an old AM radio between the antenna and the radio will help, especially if you connect the wire directly to the radio. Many commercial recievers used this technique, called "Antenna peaking" or similar, back in the tube era.

 

[dxho]

I put up a ~150 piece of copper wire, connected to a 50' underground run (under the driveway) of RG-6 to a grounding rod, then 50' to my radios. The shield is grounded to the grounding rod, all the radios, and the house electrical service, per code(s).

1) The antenna input is extremely quiet. Even though the radios are 10' from the house breaker box in the garage, and a foot or so beneath the CRT TV, they pick up no buzz. On some bands, the noise floor is right down at S1 on the Drake R8.

2) I originally used a balun to connect the wire to the coax. Eliminating that caused no drop in signal strength. In fact, it has so much signal, I had to put two attenuators on the input, since it overloads everything but the Drake.

To me, the critical factors (at least for BCB) are length and good grounding. I found I needed at least 150 feet to get good signal on BCB. Shortwave listening requires less, the higher the frequency.

I'd look at better grounding in your setup. The rest might be okay. I think you've got enough length.

 

[Big 121]

For SWL'ing, @ 8mhz, a 50 ohm 1/4 wave is approx 29'.Your setup is 100's of ohms impedance. Using such a longwire for SW tends to pick more
noise over many freqs.(man made + natural) and a much longer wire has higher losses, the higher the freqs. If you can use a vertical
wire on the roof (or slanted) maybe 25' to 50' to a support, using a coax at the roof and shield grounded in the room, this should give plenty of usable signal, while shielding the lead somewhat from the house's noise field as it comes from the roof.Connect it to the whip antenna.
If you have some money to spare, a SWL Antenna preselector(MFJ) or Active Antenna(MFJ,Kaito) will give amazing results with ULR's.
Ham Radio outlet; Universal Radio, and Amateur Electronic Supply carry these...

 

[Icangelp]

I put up a ~150 piece of copper wire, connected to a 50' underground run (under the driveway)...

I assume you have a concrete or blacktop driveway. Did you install your wire before the driveway was installed?

I am considering relocating my outdoor loop, but will have to run my cable under the driveway, and have yet to figure out how to do it.

Anyone have ideas?

 

[K4WRF]

Most important, make sure you have a *VERY* good ground. Long wires also need something to push against, hook up to water pipe if is something other than PVC. Ground rod might not be enough. MFJ Enterprises sells a tuner for SWL and DX antennas I think.

Best regards,
w/

 

[K6JHU]

"I am considering relocating my outdoor loop, but will have to run my cable under the driveway, and have yet to figure out how to do it."

Assuming that you have dirt or something soft on each side of the driveway, here is was I did.

1. Go to home deport and buy a "driveway tunneling kit", 10' of PVC (sized to match the kit), and a can of glue.

2. Glue the kit to each end of the PVC and let the glue set.

3. Dig a thrench about 1 foot deep on one side of the driveway and a hole on the other side.

4. Connect the PVC to your garden hose.

5. Lay the pipe in the trench, turn on the garden hose and slowly push the PVC level under the driveway. BTW, did I say this was more than a little messy

6. when you are through to the otehr side, let things dry out. Then slide a piece of PVC with your cable through to the other side.

With respect to the previous comments, a good ground (connected to shield on the coax) is a requirement.

 

[dxho]

I put up a ~150 piece of copper wire, connected to a 50' underground run (under the driveway)...

I assume you have a concrete or blacktop driveway. Did you install your wire before the driveway was installed?

I am considering relocating my outdoor loop, but will have to run my cable under the driveway, and have yet to figure out how to do it.

Anyone have ideas?

K6JHU gets down to it, for concrete or blacktop. Mine is gravel, still a huge pain. But like you, I had no other option. I just buried outdoor grade cable straight in the dirt. 6 years on, no problems.

I had no other option because I wanted the actual antenna away from the house. It was worth the effort.

 

[nocomradio]

Wow!

Thank you all for all the excellent replies and information!

I think I will begin by shortening the wire, changing to copper and getting a good grounding system in place. Last night's listening wasn't as noisy in the background, but I can't see anything that has changed in my immediate compound. Again, I am pretty rural, so my closest neighbor is around 1 mile from me.

I do keep one flourescent light on all the time in my shop next door, but it is constantly on and turning it off seems to have no effect. My furnace runs when it needs to, but again running or not doesn't make much difference. I haven't checked though the power supply coming into my house as I suspect that may be some of the source of some noise too. I do have a transformer in the far corner of my yard. My landline also tends to be somewhat noisy. The electrical and phone lines are grounded properly, as when I built the house I installed what was required by code and drove those mega-long ground rods in myself.

 

[K6JHU]

If you are rural and in MON like I am, improperly wired electric cattle fence controllers can cause interference.

 

[Big 121]

If your shortened antenna is around 50' , use the same wire... electrical resistance is only around 5 ohms-insignificant
for receiving. But not in transmitting.

 

[nocomradio]

Ya know, there is a cattle operation a mile or two away with a few electric fenced pastures. Never even considered that. I'd bet those things are noisy. (electrically speaking)