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I recently built a vertical. It's 6M radiator, a coil, and 1.2M more of radiator. It sits on my roof, which is metal, and acts as a ground plane (GND from the feedline is connected to it). There is a choke right at the feedpoint. The impedance is matched (with a LC circuit) from 36 ohms to 50 (49.9, for a SWR of 1.02:1 according to my VNA).

But whenever I transmit more than 50W, my internet and the neighbor's, drop. Sometimes it's a small packet loss, other times is continuous over several seconds.

This doesn't happen when I use the dipole.

The dipole is located about 15M from the street (where the cable companies cables are located, overhead), and the vertical is only 5M away from it. My cable drop rests on the metal roof that acts as a ground plane (I lifted it about 50cm but nothing changed).

What can I do to avoid this situation?

I also have a problem with strange interference near 145mhz that doesn't go away even when the power goes out. I suspect it's CATV related, so I'm thinking a leaky CATV amplifier is causing interference, and I'm causing interference to it as well.

I tried choking the cable to the modem but nothing changed.

hjf
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  • Can you verify (by connecting an ethernet cable) whether it's the modem/router or wifi connection dropping? Solutions will be different. – Zeiss Ikon Apr 06 '20 at 16:22
  • In a nice world, you'd be able to call the cable company. They'd dispatch a knowledgeable, helpful technician that would show up on time and work with you to identify where the RF is getting into their systems and mitigate the issue. But I don't think that will happen. – Phil Frost - W8II Apr 06 '20 at 16:23
  • @ZeissIkon it happens both on cable and over wifi. – hjf Apr 06 '20 at 16:38
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    @PhilFrost-W8II In the real world, they'll send out a "technician" who knows nothing about radio and not much more about internet, and who will swap out the modems/routers without any testing and bill the ham an outrageous amount for the hardware, mileage, and time. – Zeiss Ikon Apr 06 '20 at 16:53
  • @hjf So that means its in the modem or router. I agree, this is most likely related to your 145 MHz interference. – Zeiss Ikon Apr 06 '20 at 16:55
  • Weird. I put 100W FM into the antenna and the internet doesn't drop out now. I guess it could be either the cablemodem switching to another, less noisy channel, or weather-related changes on the antenna ground plane (last night it rained. now it's very windy), the ground plane is corrugated sheet metal with some overlap. It's not solid, soldered, or welded. – hjf Apr 06 '20 at 20:42
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    Interesting. I'd guess it's the fact that you're using your roof as the groundplane. With enough feedline between your tx and antenna, and a probably even longer connection of your roof to ground, you might be feeding RF into the internet equipment's ground that way.

    My second guess is your ISPs are using unshielded twisted pair for their last mile cables and catching tons of interference that way

     – Ivan R2AZR Apr 06 '20 at 22:58
  • However, aerial transmission lines tend to be horizontal while your antenna is vertical, and the dipole is harmless despite probably being horizontal... How is your dipole oriented? Are the cables (or anything else that might get zapped) in its nulls maybe? – Ivan R2AZR Apr 06 '20 at 23:09
  • One problem with using roofs and gutters and downspouts for antenna elements and ground planes is corrosion. One rusty connection can generate enough harmonics to light up the neighborhood. – Duston Apr 09 '20 at 13:44
  • You need to contact the cable company and get them to clean-up their fittings! After all, the "cable" should be a closed system! – quintablet Jul 04 '22 at 02:58
  • A horizontal groundplane can act as a horizontal radiator. Just saying. – user10489 Jan 07 '24 at 16:06
  • I would build a magnetically shielded small loop for 145MHz and maybe also your 6m transmit frequency and do some direction finding (with an AM or SSB radio) before I called the cable company. Sometimes if you can instead of saying "your stuff is leaking" say "there's a problem with the amplifier on pole XXXX, would you like me to call FCC?" they become very helpful. – user10489 Jan 07 '24 at 16:07

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