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

W9IQ

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #15 on: February 16, 2021, 07:41:47 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

That latter, Scott. The reason for the 12 feet or so of coax in the clear is to provide an effective radiator as the other "half" of the antenna. The coax that makes up the choke doesn't count towards that 12 feet because the current on that section is subjected to high impedance so it won't be as effective of a radiator. It is also coiled up so that limits its radiative effect.

You could achieve the 12 feet by leaving a 12 foot or so pigtail on the antenna side of the common mode choke. You would then avoid the extra connectors and the need to waterproof them. But keep in mind that RG-400 is fairly expensive so you need to make the economic call for yourself.

If, on the other hand, the antenna is a center fed dipole then the first choke can go right at the feedpoint because you don't want any part of the coax to contribute in a meaningful way to the radiation or reception of RF.

- Glenn W9IQ
« Last Edit: February 16, 2021, 07:47:15 AM by W9IQ »
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- 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

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #16 on: February 16, 2021, 07:52:49 AM »

Glenn, I was thinking of my receiving antennas. The feedlines are on top of the ground, so less connecting and waterproofing is better!
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W9IQ

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #17 on: February 16, 2021, 07:55:06 AM »

That sounds like a good plan, Scott.

- Glenn W9IQ
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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

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #18 on: February 16, 2021, 10:25:50 AM »

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?

The latter, Scott. The reason for the 12 feet or so of coax in the clear is to provide an effective radiator as the other "half" of the antenna.

I believe when you're saying "other half" of the antenna you're referring to the Alpha 10-160m HF JPole Sr, correct? For everyone's benefit (and I'm trying to keep this straight myself), you made this comment a few days ago:

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.

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.

I did experiment with different counterpoises and they had no effect on the background noise. Without some major re-design and location of the shack, I'm probably going to try keeping the antenna location for now, which is supported by an 8 ft non-conducting mast clamped to the main sewer vent stack on the roof. The other end is tied off to a tree at the edge of the property.

As you saw in that photo of the kludged CMC, the feedpoint is just inches (< 1ft) from the top of the mast, so my run is ~ 9 ft down the mast to the roof, then ~ 12 ft across the roof before it runs ~ 5ft down (6-7 ft including drip loop) to enter through a plywood insert in the window of the garage shack. Here's another photo taken standing outside the window of the garage shack where the feedline enters, looking up at the feedpoint of the Alpha (the 2m/70cm JPole is seen to the right). So you see, I don't have a lot of space out there. Your comment referred to coax "in the clear" ...
The latter, Scott. The reason for the 12 feet or so of coax in the clear is to provide [. . .]
...but I don't have 12 ft in the clear. 12 ft from the feed point and I'm laying on the garage roof, and the entry to the shack is just several more feet away. I've got 8-9 feet in the clear, though.

What's special about this 12 ft? Would I maybe want to put the CMC at the base of the roof mast? Or what if I put it at the top, right at the feed point, where I installed my kludgey CMC? Or is all of that ~12ft of coax laying on my roof and the ~5-7 ft hanging off the roof before going in the window an effective radiating part of the antenna, and I would lose something by choking elsewhere?

In any case, I think I would NOT build any additional lengths into the CMC, but keep those as short as possible because that makes them the most versatile. Then one can always adjust lengths and distances from feed points, entry points, etc. by means of jumpers, leaving the CMC equally usable at any location.

If, on the other hand, the antenna is a center fed dipole then the first choke can go right at the feedpoint because you don't want any part of the coax to contribute in a meaningful way to the radiation or reception of RF.

And when you refer to two chokes, I assume that'd mean two sets of chokes? For example, in my case of wanting to cover all bands I would have three differently wound chokes in series; you're suggesting a second set of three? I may just start with one set of three and see how that goes, location To Be Determined.

Dave
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KC9QBY

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #19 on: February 18, 2021, 05:40:59 PM »

FWIW, on the KF7P product, I reached 15 turns and 1ft remaining pigtails on each side with PL-259 fitting thru the toroid, a bit tighter fit on later turns.  I did use some extra zip ties for anchoring as the winding progressed slowly and cautiously.

73, Chuck  KC9QBY

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
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73,  Chuck  KC9QBY

N3WTK

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #20 on: February 18, 2021, 07:08:35 PM »

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?

Answering my own question . . . I got a message back from K9YC who says "The lengths in the table assume 4-inch leads on both ends of the choke."

I also got a message from Alpha and should probably make clear their guidance for this antenna, in which they state:
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.

I tried a grounded counterpoise with no success.

Now, I've ordered some ferrite toroids and RG400 so I'll go ahead and wind a CMC according to K9YC's cookbook for this antenna, then try also (again) adding a grounded counterpoise with the proper CMC in place. Then I will have properly tried all of Alpha's suggestions.

I don't need to win any contests here; I just would like to hear a few stations other than the handful that come in at S9+ that make it over my noise floor.  But if a proper choke and counterpoise still don't help my particular installation then it will probably be time to re-wind the choke to feed a 10/20/40m parallel dipole.

Stay tuned . . .
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Dave / N3WTK

W9IQ

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #21 on: February 19, 2021, 02:45:12 AM »

That's good to know the pigtail length for future reference. Thanks for posting that.

As I said earlier, I think Alpha's instructions are misguided. Even if their alternate techniques "worked" there is no reason not to use common mode chokes. Belt and suspenders...

Looking forward to hearing of your results.

- Glenn W9IQ
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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

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #22 on: February 19, 2021, 10:21:37 PM »

No update on the CMCs yet since the ferrites are still on order, but before we had this discussion which lead to CMCs I had already ordered an MFJ-1026. It arrived today so I set it up. I felt like it was more appropriate to put that in it's own thread so I started a new post for it here, to keep the 1026 discussion separate from this one.
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Dave / N3WTK

N3WTK

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #23 on: February 27, 2021, 10:39:08 PM »

Looking forward to hearing of your results.

Glenn, et al.,

I have results!

First, I have to say that I messed-up the "scientific experiment" a bit by switching radios. I had not imagined that would make such a difference, but it kind-of did. You see, I'd been eyeing the FT-991A for awhile and finally decided to upgrade to it. My 897D sold quicker than I thought; it sold and the 991A arrived all quicker than Mouser could ship me the toroids. The funny thing is, that my background white noise was somehow less with the new radio. Whether that's due to a different day, time, or the radio, I'll never know for sure.

On to the Common Mode Chokes...

I purchased a pre-made pigtail of RG-400 with a PL259 on one end and an SO239 on the other; this is ideal for making a CMC because then you don't need a barrel connector. It is no problem to wind any number of turns of this through a Fair Rite 2631803802 toroid; there is plenty of room for the connectors.

I decided on only two, instead of three series CMCs and using the same jumper wound a 10T and a 14T choke in series. See this photo here.

I also made one for my cable -- I get my internet through the local cable TV provider. So I also bought a jumper of RG-179 with a female F on one end and a male F on the other, wound a 22T choke and inserted it in-line where the cable connects at the front of the house.

I did not test the noise with the cableTV CMC in/out, but I did test the noise with the Alpha Antenna's CMC in/out. Direct-fed the white noise was S6-S7. Insert the proper CMC that I just made and the white noise dropped to S1. See here, if you'd like.

In one of the earlier posts I think you said Glenn, that if I had a CMC then I'd need a counterpoise. How would I know if I needed one? I did try adding a ground wire connected to the sewer vent pipe (which ultimately reaches earth and the house's electrical service ground). If I connected it to the rig-side of the CMC then it did nothing to the white noise. If I connected it to the antenna side of the CMC then the white noise jumped back up to S7.

I haven't tried transmitting yet with the CMC in place. But so far, on receive, it seems to work great!

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Dave / N3WTK

W9IQ

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #24 on: February 28, 2021, 04:03:59 AM »

That is great news. I am glad it is working that well for you.

With regard to a counterpoise, my advice is generally to allow a length of the coax to act as the counterpoise by placing the common mode choke 10 feet or so away from the matching transformer.

- Glenn W9IQ
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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

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #25 on: February 28, 2021, 08:07:54 AM »

With regard to a counterpoise, my advice is generally to allow a length of the coax to act as the counterpoise by placing the common mode choke 10 feet or so away from the matching transformer.

That is certainly easy enough to do . . . I just insert a coax jumper.
But how would I know if that is needed, or helpful in this case? Is there any way to know if it's making a difference? Would you expect a difference in receive noise? Or does it just make it a "more effective" antenna; maybe my signal gets out better? (which requires a patient, distance receiver to work with me to determine)

Part II of the counterpoise question: What's special about 10 ft?

The feed point is elevated ~9 ft above the roof. If I inserted a ~9 ft jumper it would all remain vertical and not have to lay on the roof.
 
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Dave / N3WTK

W4JFA

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #26 on: March 01, 2021, 05:04:39 PM »

I'm paying close attention since I have  intermittent noise on 20 meters using a 3 element yagi.
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W9IQ

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #27 on: March 01, 2021, 05:16:37 PM »

The antenna circuit requires a current return path. If this didn't exist the antenna feed point impedance would be infinity. By providing a relatively efficient return path through 10 or so feet of coax braid, the circuit is more efficient so the antenna gain is increased. Less efficient paths would be through trees, aluminum siding, the ground, through the choke, etc.

The 10 or so feet should generally be sufficient when dealing with an end fed half wave (or its harmonics). The better the common mode choke, the greater effect that this length will have on SWR. But most models show that the common mode choke is far from perfect so in the case of an EFHW antenna, the 10 feet of non hindered braid is not overly critical.

You can experiment with the 10 foot length to find the optimum transmit SWR across the different bands of operation.

- Glenn W9IQ
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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

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #28 on: March 01, 2021, 08:13:52 PM »

The antenna circuit requires a current return path.

[snip]

... in the case of an EFHW antenna, the 10 feet of non hindered braid is not overly critical.

So is this really an EFHW antenna? I know it kind-of looks like one. But there are two wires leaving the matching unit (whatever it is). Alpha calls it a "JPole". While typing this I decided to look up the definition of a JPole antenna. In short, Wikipedia defines it as "an end-fed omnidirectional half-wave antenna that is matched to the feedline by a quarter wave parallel transmission line stub." I've asked Alpha in the past how this antenna works exactly but as helpful as they've been I've not gotten a satisfactory answer to that specific question; they just said that it's all described on their website.

I'm in a parallel email conversation with them on this subject of common mode current and noise, and I asked Alpha Antennas what they thought of the 10 ft of coax and here is what they have to say on that subject:

     It would make most sense to have a coax that’s slightly longer than 1/4 length of 40 meters. So 33’ would be most reasonable, which would also be 1/8 on 80M, 1/2 on 20M, and a full wavelength on 10M. This would then be place place to put a choke BALUN. Alternatively, a 33’ grounded counterpoise wire clamped onto the barrel connector of the JPole would do the same thing.

Note that I had already tried grounding the coax shield at the feed point, with the good CMC installed, and that instantly brings the noise up to S9. Now, this wasn't exactly 33' (I don't remember the exact length) but I know I tried some different lengths and they just made things worse.

You can experiment with the 10 foot length to find the optimum transmit SWR across the different bands of operation.

I don't have a good variety of jumpers but I have a couple of lengths of RG8X, both about 14-ish feet long so I put one in at the feed point and put my CMC at the end of that. It made my noise go back up, so this doesn't seem to be the answer. And tried grounding the coax shield, again to no benefit.

At the moment I'm operating with only the CMC at the feed point, which seems to be the best situation (at least on RX) -- I guess it could be possible (?) that this is at a sacrifice of TX? But if I can't hear stations over the RX noise then I can't work'em.
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Dave / N3WTK

W9IQ

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Re: "Good" antenna noisy, "bad" antenna quiet
« Reply #29 on: March 02, 2021, 02:40:10 AM »

If you have a working solution then use it.

The technical commentary from the manufacturer is, in my opinion, worth ignoring. It sounds as if they been taken over by marketing folks.

- Glenn W9IQ
« Last Edit: March 02, 2021, 02:51:09 AM by W9IQ »
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- 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.
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