Crazy AM Overload with Longwire Antenna

[ultravista]

My wire antenna has a vertical run of about 30 feet then angles back down, approximately 34-40 feet to an anchor point. The first 20 feet is galvanized fence post. The last portion is PVC. The wire has a few turns around the galvanized pipe and PVC. The wire is clipped to the center threaded pole of the RTL-SDR magnetic base antenna.

I get heavy AM bleed-over everywhere throughout the HF dial using an RTL-SDR V3 dongle sampling the Q-Branch.

How do I get rid of the AM bleed-over? It is terrible and not present at all using a shorter wire thrown over my neighbors tree.

Would the metal pipe, or few turns around the pipe, have anything to do with the AM overload?

 

[boatbod]

My wire antenna has a vertical run of about 30 feet then angles back down, approximately 34-40 feet to an anchor point. The first 20 feet is galvanized fence post. The last portion is PVC. The wire has a few turns around the galvanized pipe and PVC. The wire is clipped to the center threaded pole of the RTL-SDR magnetic base antenna.

I get heavy AM bleed-over everywhere throughout the HF dial using an RTL-SDR V3 dongle sampling the Q-Branch.

How do I get rid of the AM bleed-over? It is terrible and not present at all using a shorter wire thrown over my neighbors tree.

Would the metal pipe, or few turns around the pipe, have anything to do with the AM overload?

I presume by "AM" you mean am broadcast frequencies? If so, install a high pass filter that cuts off before the MF band.

 

[ultravista]

Yes, the AM broadcast band. Are the filters precise, meaning, will it only filter x to y allowing above and below to pass?

 

[ultravista]

Also, can someone explain what causes overload from local stations when using a long wire? Does the long wire become a very efficient receptor? On other words, what specifically about a long wire makes the receiver overload?

 

[ka3jjz]

It's not so much the antenna as it is the stick. You may think of the stick as having a very wide gate with no blocks in the way to herd things away. There's little (if any) filtering and buffering that a standard radio would have (keeps costs down), so too long of an antenna is simply overloading the stick.

Yes different kinds of filters exclude (or pass) frequencies while attenuating others. There are many different kinds of filters, and in the case of a MW (AM band) filter, you should be aware of the frequencies that are causing you issues. Without getting too deep here, you must be aware of the cutoff (the point at which the filter stops working, and passes RF with little or no attenuation) and where the filter begins to work. The better filters - which will cost more - have a graph where you can actually see what the filter's response would be. It only takes a moment of study to understand it; this will help you determine whether the filter will actually work (in theory, of course - your environment has much to do with how the filter actually performs) for your situation.

Here's a very simple test you can perform. Take the antenna off completely and see which MW stations still come in at a strong level. Take notes of the frequencies - these are the ones you want to make sure are attenuated. Mike

[edit] One other thing - before going this route, try reducing your gain. Too many folks think that you need to run your gain up all the way. This is patently false. What you need is the gain to be able to hear the station **with a minimum of noise and other interference** This can only be found by spending time at the dials (so to speak) and playing around with the settings.

 

[RFI-EMI-GUY]

Back when I was a shortwave "Newbie", in about 1979, I strung up a "long wire" antenna using a small conductor and extended it to a tree in a far corner of my lot.The near end was wrapped around a nail and extended in a window, Being concerned about lightning I would disconnect it from my Kenwood R1000 receiver when not in use. One day I came home after a huge storm and found my doorbell, a homemade device using CMOS chips to replicate the urgent message jingle from "The Presidents Analyst" , was stuck on one note. Several other devices in my technologically sparse household were likewise damaged. My long wire antenna had no lightning protection, no ground return yet it induced a heavy current into the electrical mains of my house.

The lesson here, if you are going to fool around with SDR, you should get serious about lightning protection, antenna surge protectors etc,, or you are going to let in a lot of heavy current into your USB port and god knows what else in your modern home.

 

[RFI-EMI-GUY]

Also, can someone explain what causes overload from local stations when using a long wire? Does the long wire become a very efficient receptor? On other words, what specifically about a long wire makes the receiver overload?

Partly it is resonance, but mostly the proximity to strong transmitters. You are far closer to many medium wave "AM" stations than any shortwave stations. My present house has an AM station close by that would be superimposed on the telephone answering machine recorder. I could not hear the station when making a call, but sure enough, on recorded messages it was loud. So I detuned the wiring with some chokes and eliminating unused phone wiring.

 

[ka3jjz]

A typical random wire - a true longwire (notice it's one word, not 2 as so many tend to write it) is a very different animal - of any length less than several hundred feet long is nowhere near resonant on any MW frequency. Even at 1 Mhz (1000 Khz), a quarter wave is over 200 feet. But even so, any nearby MW station even on a 100 foot random wire is going to absolutely saturate the front end of these USD20 sticks. These sticks simply can't handle that level of RF because they have little or no filtering on the front end. Either reduce the gain, or if that doesn't work, a filter will be needed.

It's bad enough when it's just one MW station, but if you are unlucky enough to have several in your area, you are likely to need a filter- gain adjustments are likely not going to be enough.

Mike

 

[RFI-EMI-GUY]

My wire antenna has a vertical run of about 30 feet then angles back down, approximately 34-40 feet to an anchor point. The first 20 feet is galvanized fence post. The last portion is PVC. The wire has a few turns around the galvanized pipe and PVC. The wire is clipped to the center threaded pole of the RTL-SDR magnetic base antenna.

{snip}
Would the metal pipe, or few turns around the pipe, have anything to do with the AM overload?

The galvanized portion is going to create intermodulation distortion if you actually have copper wire touching it. If it is an insulated coil of wire around a metal pipe, you have unknown stray capacitance and inductance affecting its resonance.

There are so many good sites regarding antennas and grounding practices that I suggest you research them and work on improving the grounding and design of the antenna. You might also want to build a preselector filter so that you can tune the receiver and limit the bandwidth to what minimum you need.

 

[KC4RAF]

+1 with what 'ka3jjx' posted in his edit about your gain.
Many years back a local ham explained how this works. The idea was to turn the gain down and the volume completely to max. Then use the gain as you would the sound volume knob, starting at the minimum level. That was for listening to the sidebands, but found out that it works great in bringing in station with a reduction of static in the AM mode as well.
hth

 

[ka3jjz]

Well that technique works well on tabletops, it's not quite the same on a SDR. But the principle is the same regardless. Don't over amplify a signal beyond the point where its listenable - and like I said that takes time on the dials (sort of) to learn this.

The suggestion about preselectors is a very good one - however the construction techniques needed might be a bit beyond a newcomer (wrapping toroids and making sure good shielding is maintained is crucial). However there are 2 commercial units I know of, and even with the Euro conversion and customs, I suspect the one from the UK would be a bit less expensive than the MFJ unit

MFJ Enterprises Inc.

Cross Country Wireless HF Preselector

This question has come up before; perhaps a wiki article branched from the SDRs on this subject is in order here...Mike

 

[ka3jjz]

This is advertised as a 'preselector / antenna tuner' (hmmmm..); it's a good bit less expensive than either of the 2 units above...without seeing a schematic, it's impossible to tell - but I wonder if this is like the Cross Country unit?

https://www.mfjenterprises.com/Product.php?productid=MFJ-956

Mike

 

[ultravista]

Thank you all for the informative responses.

 

[k9wkj]

for my setup i run a broadcast band notch filter
and a SV1AFN hf upconverter and i still must keep the dongle internal gain way way down
this isnt just a SDR problem
BCB interference plagues every kind of radio
i was reading a article about a fella setting up a 160m vertical and he was measuring over 5 watts of energy coming back down the line from a local broadcaster, he had to design uber filters and traps to dump that to ground to keep his receivers from getting swamped

 

[spongella]

You can also reduce the RF gain on the dongle or get yourself a shortwave antenna tuner. First try lowering the RF gain though.