Antenna overloading question

[modrachlan]

I've thrown a random wire- about 100 ft- down the yard for in a east-west orientation with the idea of experimenting and eventually setting up a permanent longwire antenna. Attaching it to radios- clipping it directly to the whip, or wiring the lead to the center conductor of a variety of connectors, e.g. 1/8" mono jack, 1/8" stereo jack, and PL-259, results in overloading- everything from a Tecsun PL-310 to a Lowe HF-150. All across the spectrum I hear a strong local AM station in the background. So I'm trying to control that.

I've made a few changes. Stage one: buying a 9:1 unun kit and mounting it in a box, then driving a copper pipe into the ground, clipping that to the ground wire of the unun, attaching the long wire to the antenna lead wire, and running the receiver wire to the radios, clipping and plugging, etc. No improvement. Stage two: adding an active preselector in an attempt to suppress the AM station. No improvement.

Now, here's where it gets weird, or I betray what little I know about antennas. I was testing the longwire at the opposite end- I wanted to see where the performance seemed best: at 4 ft (the highest I can get it for now), about a foot off the ground, or on the ground. (i've read about the grass longwire and thought I would give it a try to reduce noise.) On that end, I don't get any overloading when I splice the wire with a 1/8' stereo plug and a short lead soldered to the center conductor. (the height test seemed inconclusive- sometimes 4 ft was better, sometimes on the ground was just as good as not). I did get good gain- almost as if I connected things backwards.

I followed the instructions for the unun closely, so I'm pretty sure it's connected to the right wires. I'm open to having done something wrong, though. I'm wondering just what the heck I need to do to suppress the strong station. I thought the preselector would make a difference, but really seems to make none at all.

If I am going about this all wrong and just need to do something else, by all means let me know.

Thanks for any insight.

 

[Boombox]

All three radios you mentioned (even the Lowe) are famous for overloading on long antennas close to strong AM transmitters. The Tecsuns overload on long wire antennas because they're portables and designed to work off a whip or a shorter wire.

The Lowe just has too much gain for areas with a local AM station around (I've never used a Lowe, just read that in WRTH not too long after the Lowe 150 came out. That's why they built the Lowe preselector, to help contain some of the overload.).

I'd shorten the antenna, or try using the portables' whips and use the antenna with the 'near field' method, i.e. set the whip near the end of the antenna without touching it. You could also try to loop the lead in wire around a tube and set it near to the radios, the near field effect may help in that way.

That safety gizmo that one of the guys here talks about has a built in potentiometer, as well as neon bulbs / diodes for safety from static electricity. Perhaps that circuit would help, because of the potentiometer.

Or you could get a DX-398 / Sangean 909 (or similar radio) which is more capable of strong signal handling. I'm the last person here to say "get a better radio". I use digital portables. But with certain radios you're going to have overload. There may be a way to overcome it with a high pass filter, or a notch filter of some type. Or maybe try a different kind of antenna.

I don't know why your antenna worked differently from the other end. If you really wanted to find out whether there truly was some sort of electrical difference off each end of the antenna I'd temporarily remove the Un-Un and see if the difference is still there.

I'm sure the antenna geeks here will have a lot better ideas than I have.

PS -- does the same thing happen with your Wellbrook? Or is that not a SW antenna... (not that up on Wellbrooks, just read about them on the forums).

 

[ko6jw_2]

There is a very strong possibility that your receiver is overloading. Many small receivers do not have the necessary front end selectivity to handle strong out of band signals because they were meant to work with short whip antennas. The preselector is a good idea, but the signal may be too strong for it to work well. You should consider an attenuator to reduce the signal getting to the receiver. The steps you have taken may actually improve the antenna and make the overload worse.

In terms of antenna design an antenna is "long" only when it is several wavelengths long not just a long piece of wire.

 

[mm]

http://www.dxing.com/tnotes/tnote06.pdf


A Broadcast band notch filter or SWL high pass filter properly connected and grounded as in the above article.


The high pass filter methods effectively blocks the AM Broadcast energy going to your SWL receiver

 

[ka3jjz]

The preselector would have been the answer if it wasn't an active type (which has additional gain, which in this case, you don't need). There's another kind of preselector, known as a passive preselector, which has no amplifying circuits, but acts as a 'tunable gateway' of sorts, adding front end selectivity to radios that don't have it. They're rare today - as far as I know, MFJ is the only one making them anymore. In addition to Lowe, another company - McKay Dymek - made these as well, but they are, sadly, long gone.

However winding your own coils and such can be a daunting task for a newcomer. There are several places where you can buy a filter that will cut off the AM band. The trick here is to know precisely what frequency (ies) are causing the issue. Once you know that, you can determine which filter will work the best for you.

See the very bottom of this wiki page (links are always blue)

HF Equipment and Accessories - The RadioReference Wiki

The best way to find your offending station is to use the 150 without any connections to antennas or grounds (it's best if you could run it off the batteries, but if you can't, just be aware that in some situations, the AC cord can act as an antenna as well). Take notice of the MW stations that you hear, and note the signal strengths. Plot them out so you have a list. Then take a look at the offerings above and see which ones will suit your needs.

I'm sure there are other manufacturers out there, and any additional links that addresses this are welcome.

Mike

 

[modrachlan]

Speedy and thorough replies... I appreciate it. Thank you so much.

I will conduct the MW experiment with the Lowe and go do my reading about bandpass and attenuation.

 

[a29zuk]

When you connect your radio to the opposite end of the antenna, are you using batteries only to power the radio? Or a different outlet in a pole barn? The electrical ground(not antenna ground) to your receiver can act as part of your antenna circuit(i.e. hot water or drain pipes can run throughout your house). There is a nearby AM station at my location so experimenting with the electrical ground or eliminating it(using batteries) has helped with overloading on some of my receivers. Especially on ground soaking rainy days.
I don't know what your exact situation is... but just a thought.....
Good Luck,
Jim

 

[modrachlan]

just batteries on both ends. same radio. this is why i was so confused.