Pages: [1] 2 3   Go Down

Author Topic: "Good" antenna noisy, "bad" antenna quiet  (Read 493 times)

N3WTK

  • Posts: 84
"Good" antenna noisy, "bad" antenna quiet
« on: February 14, 2021, 01:20:49 PM »

This is partly a post about RFI, partly about antennas.

I just installed a brand new Alpha Antenna 10-160m JPole Sr (https://tinyurl.com/2x9slkvu) at my suburban Los Angeles QTH. The feed end is elevated and sloped down as per manufacturer's instructions, and SWR is pretty good, as advertised. This is my first time on the air at this QTH.

On all bands I have a continuous white noise at S8-S9, everywhere, all(*) the time. (*="all the time" is not exhaustive; I have not systematically checked yet at all times, but every random time I have tuned in, the noise is there.) I started reading articles and watching YouTube videos about tracking down RFI sources, etc. I shut down the power to my entire house at the main breaker, operated the FT-897D from a battery and saw no change, so it doesn't appear to be anything that I am generating. I was struck by one HAM who found his noise source was a wall wart in his barn, 40 feet from his antenna. As I am surrounded by neighbors, any one (or all) of them could have any number of wall warts (or plasma TVs or grow lights, etc.) and I am not going to be able to help that.

I decided I would try the MFJ-1026 Noise Cancelling Signal Enhancer. It is still on order, due to arrive next week.

Meanwhile I'm setting up the aux noise antenna that will be used for the 1026 noise reference. One HAM I saw (https://youtu.be/ott_k_C2b_U) used the Russian made active MiniWhip antennas so I thought I'd give it a try. My miniwhip (https://transverters-store.com/whipinbox.htm) is installed on a plumbing vent pipe using a wooden stand off so that the PCB is above ground, as instructed. There is a wire to the PCB ground plane that I can connect to the vent pipe (instructions say this antenna must be high and must be grounded).

Now here's the thing-- if I leave the miniwhip ground wire disconnected, then I have the SAME RFI (same white noise sound, same S8 level) as my AlphaAntenna. But if I connect that ground wire as instructed, the noise on the miniwhip completely vanishes! *AND* I hear signals! Switching back and forth between the Alpha and the (grounded) miniwhip, I can pick up the same stations, only better, because there is no noise. So this $23, 7-inch long "junk" antenna appears a better receiver than my $200 60-ft "good" antenna.

I don't have the MFJ-1026 yet so I don't know yet how well it's going to work with cancelling my noise but clearly it seems I'll want to operate the miniwhip ungrounded, since that seems to allow it to pick up the same noise, and if it's quiet then it's probably not going to be a good noise reference for the 1026. But the fact that connecting the miniwhip ground wipes out the noise has me more curious as to what the noise mechanism might be here, that grounding could make it go away. Does that mean that I could do something to my AlphaAntenna to make its noise go away? Maybe it's something peculiar to the miniwhip that I don't understand.

Incidentally, linked here (https://postimg.cc/Wt4z7DNR) is an arial view of my QTH antenna farm. North is up, East is right. The red line (0) is the AlphaAntenna. The feed end is supported with a non-conductive 8 ft mast attached to a sewer vent pipe. At (1) is the mini-whip. At (2) is a 2m/70cm J-Pole mounted to another vent pipe. At (3) is a ground rod and my radio operating position.

Thanks,
Dave / N3WTK
Logged
Dave / N3WTK

W9IQ

  • Member
  • Posts: 8866
Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #1 on: February 14, 2021, 01:41:40 PM »

Dave,

You didn't make any mention of the use of proper common mode chokes on your coax. Do you have any installed?

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

N3WTK

  • Posts: 84
Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #2 on: February 14, 2021, 01:58:47 PM »

Glenn, No common mode chokes installed, neither on the AlphaAntenna feed line nor on the miniwhip feed. According to AlphaAntenna,

Quote
For the initial installation, we do not recommend a Balun or air choke coil be used at the antenna. We recommend that you ground your tuner and rig. Your radio has a ground bolt on the back of it, and rig manufacturers (and we at Alpha) do recommend an RF ground be connected on it to balance the circuit.  We only recommend that a RF Choke be used if you find one is needed, as would be the case if you hear feedback through speakers, etc. when you transmit. However, a Grounded Counterpoise would be the first option for any common mode current that might exist. If you find that common mode exists after you install the antenna, a Grounded Counterpoise wire can then be attached to the barrel connector at the antenna‘s feed point, which can be secured with a stainless steel hose clamp.

so I didn't add anything. I was thinking of trying to add a ground wire from the coax connector at the feed point, to see if that might do anything; but I mentioned my S8 white noise to Alpha and they didn't think a ground would help it. Their response was that an efficient antenna was going to pick up noise and that adding a ground wouldn't change that, but that I instead had to locate the noise source.

As for the miniwhip, I wasn't trying to eliminate noise from it -- in fact, I wanted the miniwhip to receive as much noise as possible since I was going to use it as the 1026 noise reference input.

Dave
Logged
Dave / N3WTK

W6MK

  • Posts: 4095
    • HomeURL
Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #3 on: February 14, 2021, 02:37:48 PM »

1. I decided I would try the MFJ-1026 Noise Cancelling Signal Enhancer.
Meanwhile I'm setting up the aux noise antenna that will be used for the 1026 noise reference.

2. Switching back and forth between the Alpha and the (grounded) miniwhip, I can pick up the same stations, only better, because there is no noise. So this $23, 7-inch long "junk" antenna appears a better receiver than my $200 60-ft "good" antenna.

3. I don't have the MFJ-1026 yet so I don't know yet how well it's going to work with cancelling my noise but clearly it seems I'll want to operate the miniwhip ungrounded, since that seems to allow it to pick up the same noise, and if it's quiet then it's probably not going to be a good noise reference for the 1026.

4. The fact that connecting the miniwhip ground wipes out the noise has me more curious as to what the noise mechanism might be here, that grounding could make it go away. Does that mean that I could do something to my AlphaAntenna to make its noise go away?

1. The signal phasing gadgets can work to reduce noise but in my experience the reduction is maximally 3 (6 dB each) S units. With a noise floor of S8 to S9 a significant noise problem remains.
If you do a bit of research on line, or even read the manual (available on line) you will realize that some experimentation is required to discover the type and installation of a functional noise antenna.
I've used a number of noise antennas including a directional RX loop, a low long wire and a dipole with limited directionality. Directionality in a noise antenna may be helpful, but only experiment will tell you what will work in your circumstances.

Purchasing a certain antenna and assuming it will work as an effective noise antenna is a shot in the dark.

2. The very inefficient (because very tiny) "miniwhip" with its preamplifier may give a better, and desirable, S/N ratio. So why not simply use it as an RX antenna? Use the end-fed wire as a TX-only antenna.

It's very unlikely that the "miniwhip" is a better antenna than the long end-fed wire. It's apparently
a less noisy antenna. For RX, S/N ratio is the critical factor rather than ability to pick up weak signals, other matters being equal.

3. Evidently the "miniwhip" design, with a "grounded" installation, is less subject to common-mode pickup of noise. That is a good feature of that antenna. Common-mode pickup (where the feed line is acting as part of the antenna rather than simply as a transmission line) is often a major source of noise
in an RX system.

4. You can indeed do something to reduce noise pickup on your end-fed wire. End fed wires often have common-mode noise pickup problems. The problem is that in such an antenna where a separate counterpoise is not installed, the coax feed line acts as the counterpoise and picks up excess noise.

The solution for the end-fed wire antenna is to provide a separate counterpoise wire attached at the transformer end of the antenna. Length of counterpoise wire to be determined experimentally. An internet search on end-fed wire antennas will suggest some appropriate counterpoise lengths to provide adequate tuning on multiple bands.

In addition to the counterpoise, a good common-mode choke should be added at the antenna feed point. This will prevent the coax feed line from acting as a part of the antenna and picking up excess noise.

If the end-fed wire, with counterpoise and common-mode choke were installed at the same height as the "miniwhip" the noise it picks up might be significantly reduced too.

Best bet is simply to use the quieter antenna for receive.

Logged

W1VT

  • Member
  • Posts: 6071
Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #4 on: February 14, 2021, 02:57:48 PM »

It may be that the Alpha antenna is closer to the noise source.  Many noise sources fall off very rapidly with distance.
I've improved my reception simply by moving the antennas farther aware from inhabited buildings, but I realized that many don't have this option.

I've talked to hams who say they couldn't hear anything with the antenna running over their house, as it picked up too much noise.
They also said the antenna became usable when they moved the antenna away from the house.

When I was a kid it was obvious that one could improve a station by using a shorter feedline.  Less line loss means stronger signals.
This is no longer true.  A longer feedline allowing better antenna placement can improve reception by several S-units.  You can't talk to someone you can't hear.

Zak W1VT
Logged

W9IQ

  • Member
  • Posts: 8866
Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #5 on: February 14, 2021, 03:30:40 PM »

Dave,

The Alpha Antenna 10-160m JPole Sr is an end fed antenna. Like all end feds, the obvious wire is not the complete antenna. The coax cable outer shield forms the other part of the antenna. No amount of grounding or adding of counterpoises will completely eliminate that effect.

When transmitting, the only obvious ill effects of common mode current on the outer part of the shield might be RF interference with household electronics or a 'biting mike'. In reality, the common mode current also affects the pattern and gain of the antenna.

During receive, common mode current is even more insidious. The outer part of the coax shield is part of the antenna. Since this part of the antenna is extended directly into your house, it is close to all of the RFI noise sources in your house and readily picks them up and couples them into your receiver. Trying to provide an illusive 'RF ground' as suggested by Alpha Antennas will not significantly change this.

You can experiment with a purposeful counterpoise for your antenna that may provide some relief. But you should move the antenna as far away from the house as possible and then use two proper common mode chokes on the coax. One choke should be 12 feet or more from the feedpoint of the antenna. The second choke should be located at a point before the coax enters the house. These chokes may not be the complete solution but they will go a long ways towards reducing RFI pickup by your coax. Be prepared that the SWR of your antenna may change as a result of the chokes.

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

N3WTK

  • Posts: 84
Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #6 on: February 15, 2021, 01:31:01 PM »

Guys,

First, I should be a little more fair to the Alpha. I played around a little more today and found that, while the MiniWhip had zero background noise, it also didn't pick up any stations that the Alpha couldn't. So I don't think it's fair to say that it's a better RX antenna. (Although the converse is also true -- the Alpha did not pick up any stations that the MiniWhip couldn't!) It's just interesting that I have the same "apparent" noise (same sounding, same level) on the MiniWhip when it's ungrounded, as I have on the Alpha, and that it goes away when the MiniWhip is grounded (as it is supposed to be).

Second, the MiniWhip wasn't purchased as a "shot in the dark". As I referenced in the original post, it was purchased because another HAM had the same-sounding noise as I had and he used the MiniWhip successfully with the 1026 as a noise reference antenna (See https://youtu.be/ott_k_C2b_U?t=277) which knocked out his 30-over-S9 noise. I realize that our noise sources can be completely different, so OK, maybe it was a bit of a gamble, but it seemed reasonable to try it.

Third, today I tried adding a ground to the coax shield at the feed point of the Alpha, and just like Alpha said, it had no effect on the noise. I tried various lengths. First, I tried a short (8ft) piece to the sewer vent pipe (which ultimately reaches earth ground which is bonded to my electrical service ground). I tried ~ 1/8 and ~ 1/4 wave length runs (the latter took me straight to my ground rod), and a few odd lengths in between. No length of wire, present or non present, grounded or ungrounded, here or there, when connected to the coax shield at the feed point had any effect whatsoever.

I wonder, if I put together a common mode choke, something like that shown in Figure 4 of this document (http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf), if it would have any effect? That's $40 in ferrite cores alone for just one, $80 for 2, plus coax, shipping ... I'm probably $100 into CMC just to see if it does anything, so that's a pricey gamble.

Another observation perhaps worth mentioning. The 2m and 70cm bands are absolutely quiet. Using AM mode, there is no noise in the 118 MHz, 144 MHz or 446 MHz areas. See original post for relative location of 2m/70cm JPole. The JPole is grounded to another plumbing vent pipe, and I do have three ferrite snap-on cores at both feed points. Of course, this is vertical (and so is the MiniWhip) and the Alpha is horizontal. But whatever my S8 background is, it's not picked up by a vertical 2m/70cm grounded JPole.

Dave
« Last Edit: February 15, 2021, 01:35:49 PM by N3WTK »
Logged
Dave / N3WTK

W9IQ

  • Member
  • Posts: 8866
Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #7 on: February 15, 2021, 01:51:06 PM »

I wonder, if I put together a common mode choke, something like that shown in Figure 4 of this document (http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf), if it would have any effect? That's $40 in ferrite cores alone for just one, $80 for 2, plus coax, shipping ... I'm probably $100 into CMC just to see if it does anything, so that's a pricey gamble.

That is a poorly designed common mode choke - don't waste your time or money. Take a look at the K9YC Balun Cookbook. You will get much better results using a single FT240-31 core for each CMC.

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

N3WTK

  • Posts: 84
Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #8 on: February 15, 2021, 02:38:06 PM »

I wonder, if I put together a common mode choke, something like that shown in Figure 4 (of this document), if it would have any effect? That's $40 in ferrite cores alone for just one, $80 for 2, plus coax, shipping ... I'm probably $100 into CMC just to see if it does anything, so that's a pricey gamble.

That is a poorly designed common mode choke - don't waste your time or money. Take a look at the K9YC Balun Cookbook. You will get much better results using a single FT240-31 core for each CMC.

Glenn -- great! thanks for that article; I'll read it tonight. The question remains as to whether or not a CMC will help, and I may have an answer. I just kludged something together with a 4m long RG8X jumper I found in the garage. I wound two different diameter air coils into it and taped on two snap-on ferrite cores that don't quite fit (see this image here) and as ad-hoc as this is, it dropped my noise by 1 S unit! So I'm hopeful that a better designed CMC will help.

To show if this was real or not, I set up my daughter's iPhone in front of the rig and Facetimed to my iPhone on the roof, so I could quickly swap the kludged CMC in and out while watching the S-meter and it repeatedly went down and up by 1 S unit with this kludge in and out.

(And again, attaching the ground wire lead to either side of the coax jumper had no effect.)

Dave
Logged
Dave / N3WTK

W9IQ

  • Member
  • Posts: 8866
Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #9 on: February 15, 2021, 02:40:34 PM »

It sounds like you are on the right track.

Ground connections are highly overrated...

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

KC9QBY

  • Member
  • Posts: 55
Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #10 on: February 15, 2021, 07:49:25 PM »

Plenty of noise here, now improved.  Agree you can homebrew.  That said, here is a kit that requires only winding.  RG400 31 type worthy investment in my experience.

https://www.kf7p.com/KF7P/RF_chokes.html

Good service, good products.

73 and good luck,  Chuck   KC9QBY

Logged
73,  Chuck  KC9QBY

N3WTK

  • Posts: 84
Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #11 on: February 15, 2021, 11:50:06 PM »

This is great stuff.

I wonder, could I wind three separate CMCs, each on it's own 2.4 in #31 toroid, of 10T, 12T and 17T, then connect them in series, to effectively cover the 10-160m bands? I don't see why not since the K9YC cookbook says:

Quote
Chokes in Series: In general, any combination of chokes can be used in series to provide the
desired choking impedance over the desired bandwidth. Their combined choking impedance, RS,
will be the sum of their RS values on each band.

He has various "recipes" for multi-band CMCs that have two in series; by adding a third I could cover all of the bands with at least Rs=7kΩ choking resistance. He just doesn't have a single recipe for the entire 10-160 coverage but maybe that's just because he hadn't tested it.

Glenn, do you know if the "Len" column in the spreadsheets includes the connectors? For example, in this spreadsheet for RG400, for 10T the length is 2.8ft. If I bought a 3ft assembled jumper of RG400, would that be sufficient, I wonder?

I found RG400 jumpers with a PL259 on one end and SO239 on the other end, 3ft, 4ft, and 5ft for $13, $14, and $15, respectively. And the 2631803802 toroids are only $4.75 each at Mouser.

I'm assuming I can wind the assembled RG400 jumpers through the toroids since the KF7P link from Chuck says they are in "kit" form.

Dave / N3WTK
Logged
Dave / N3WTK

W9IQ

  • Member
  • Posts: 8866
Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #12 on: February 16, 2021, 03:27:00 AM »

Dave,

My understanding of the length column is that it represents the length of the coax required for winding. It is also the basis for the calculated differential loss.

You can do a quick calculation based on the ID of the toroid and the OD of the coax to determine if the connector can pass through when nearly fully wound. I have never tried that approach but from memory only, it would seem doable if the connectors have no flanges. It might involve briefly pushing the windings to one side of the toroid.

Please report back here as to how the premade cable works out for you. It is a nice, time and tool savings concept.

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

W9IQ

  • Member
  • Posts: 8866
Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #13 on: February 16, 2021, 04:23:21 AM »

Dave,

On review, it may be that the length includes the pig tails. In the cookbook is the reference:

Attenuation is computed for the length of wire needed to wind the choke with some additional
length for leads, and is based on my measurements of each wire type for each band.


This notion is also supported by checking the winding length of an FT240-31 on toroid info with some allowance for the fact that RG-400 cannot be made to tightly conform to the core shape.

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

N2QLT

  • Member
  • Posts: 123
Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #14 on: February 16, 2021, 07:15:01 AM »

Nice thread. I noticed that the kits wind up with rather short pigtails when assembled. Glenn, you had mentioned somewhere that the choke should be at least 12 feet from the antenna. Should the RG 400 be longer in order to provide for this? Or use coax jumper and barrel connector to get it away from the antenna?


Scott
Logged
Pages: [1] 2 3   Go Up