Attic SW Listening Antenna

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radioetc

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Looking for advice on improving this existing antenna. Was considering an outside sky loop antenna but for a variety of reasons have scrapped that idea. So I want to improve this attic antenna.

The principal radio being used on this antenna is a vintage Hallicrafters with a high impedance balanced input. However, I'd like to occasionally run a Tecsun portable on this antenna too.

Right now, I have about 43 feet of uninsulated copper wire attached to PVC pipe as insulators drilled into attic studs. This length of wire is said to be resonate around 10.8 Mhz. The antenna is end fed with 75 ohm TV coax where the shield is grounded to cold copper water pipe (FYI household electrical ground is also water pipe). Also scavenged four slip on ferite beads from old VGA monitor cables and slipped all four over the coax about a foot from the radio. FYI...the coax is connected to the radio such that the center coax wire is connected to the "A" terminal, the shield is connected to the ground terminal, and "D" side of the balanced input is shorted to ground per the manual instructions for single end fed wire installation.

This antenna works but it seems it could work better.

From researching this, the options that I seem to have to improve signal and lower noise are as follows:

1. Install a 9:1 unun on existing end fed wire (such as the Winradio WR-LWA-0130 variety).
2. Cut the antenna in the middle to create a dipole and then use 450 ohm ladder line instead of coax.
3. Cut the antenna in the middle to create a dipole and use a 9:1 balun with existing 75 ohm TV coax.
4. Acquire an antenna tuner such as MFJ-16010.

Some of what I have read conflicts with other things I have read so wanted to get opinions here.

If I am understanding this right, if the radio is high impedance with a balanced input, then I do not need any baluns if I acquire 450 ohm ladder line, cut the antenna wire in the middle and thus make a dipole, and run the ladder line straight to the high impedance balanced input. Is this understanding correct? This is what I would like to do since it is cheap and relatively easy.

However, I also have a Tecsun PL-880 that I would like to occasionally use with this antenna and I presume it has a 50 ohm input. Am I going to need some transformer when using this radio if I am set up as above? In other words, what is best practice when switching radios of different input impedance?

That all said, any advice or opinions? Thanks!
 

ka3jjz

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While you could use a balun to feed the 880, why bother? Short the 2 ends of the 450 line together and the feedline becomes part of the antenna. In some cases, that's not a bad thing. Easy to try it. This was an old trick that was used many years ago. I'd fashion a pigtail to keep the strain off the antenna connector, though.

And I haven't dealt with Hallicrafters radios in many years, but yes, I believe you can feed that 450 into the balanced input and it should work just fine.

If your shack is upstairs or you're in an apartment, the cold water pipe is not likely to be a very good RF ground. It's way too long before it actually reaches earth, and may in some cases, act as more of an antenna than a RF ground. Bonding between joints is also a potential issue; you can't guarantee that you have a solid connection throughout the system.

You are very susceptible to picking up noise from the home if you put an antenna like that in the attic. There are a number of much better possibilities such as;

A good amplified loop, like the W6LVP or MFJ one, both have gotten very good reviews and are WAY cheaper than a Pixel or Wellbrook. You get the noise reduction (to a point, of course, putting it indoors is a compromise - all indoor antennas are compromises), and some directivity on the lower bands. An el cheapo TV rotor can be used to turn it - no worries about the weather doing damage. The Hallicrafters will be just fine with such an antenna, and I know I've heard of at least a few folks using the LVP on a 880.

You don't want to spend a lot? How about using the LVP Experiments's kit? Make your own loop

The Carpet Loop is a fun little project - I built one (had the antenna in my attic) and it worked better than I expected. It's still indoors, of course, but it's a winner in my book. Better than a piece of wire thrown around the attic? Yup


And don't forget the Vibroplex PAR antenna. It will work in the attic, and you have the advantage of putting more wire on it if you want. You can fool around with the matchbox to see which connection works best for you.

All of these use coax as the lead in. A common mode choke might also be useful in keeping noise out of the radios.

There you go. A few ideas; some more expensive than others, but all of them will help with the noise issue. No indoor antenna will ever beat out an outdoor one, but there are better ways to go....Mike
 

radioetc

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Hey ka3jjz! Thanks for the response!

I actually have the MLA-30 cheapo Chinese magnetic loop so I already have one of those types of antennas. And I mainly use it on the Tecsun (because both are portable). And it works pretty good!!! Sometimes the loop works better than the long wire but at other times the long wire beats the loop. I know there are better loops out there, but the cost benefit ratio of one of the more expensive loops isn't worth it. One thing I did was cut the USB power cable to the bias tee so I could wire in an old 5V linear power brick to get rid of the USB switching power supply noise. I would have used a battery instead but when I tested that it was no better than the brick. But the linear brick was better than the switching USB power supply. I also reduced the gain some on the attenuator pot in the bias tee (there's another pot in the amplfier but seems better leave that one near the factory setting). Anyway, not to get off topic but I really like the MLA-30. It's a good entry level antenna.

With the Hallicrafters, I'd like to have a permanent attic antenna that works as good as possible. When I'm in the mood to park myself in front of that radio I want to just turn the power on and it works. No fiddling with antenna (unless I am in the mood). So I think I am going to order ladder line and see how that goes. And shorting out the line for the Tecsun sounds like the way to go since I will probably not use the attic antenna on the Tecsun that much. Probably.

As far as the carpet loop goes, that is an interesting antenna idea. If I can source the parts without too much hassle or cost, I think I am going to try that. I like the tuning element of it.

Ya I know, outdoor is better. In my case, I am nervous about lightning. Also, I have power lines running across the entire back yard and running from there to the house. And places to mount and route the antenna lines is awkward. So with all those things considered, it just seemed to be too much engineering gymnastics to errect an antenna that is safe, would be low noise, and catch weak signals dramatically better than I can with some flavor of indoor antenna. Maybe as a compromise I will rig something up I can temporarily toss up when sitting outside and take down when I go in.
 
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