The AM Forum

Hi People,

I scored a Hammarlund HQ-129X at a hamfest a week and a half ago, at a great price ($60).  It's in good operating shape to boot (one of its previous owners, did some restoration work on it a few years ago).  I've decided to pair it with my Viking II.  

As some of you may remember, I live in an apartment building.  I also live in the radio Hole of Calcutta, due to 3, AM BC stations being less than 2 miles from me.   My biggest nemesis, is the "evil" WSSP (a 24-7 sports format station, that's about 1 1/2 miles southwest of me), transmitting on 1250 kHz.  I checked online, and found out that I am in the main lobe for the 5 kW signal it transmits from its 4-square antenna array.  As a result, it beats up on all of my receivers bigtime (its 3rd harmonic at 3750 kHz is typically 20 over 9 in all of my receivers, and I deal with images from it from 3600, to 3900 kHz).  

A couple of years ago (when I was using an SX-96 for an AM receiver), out of desperation, I threw up/draped a kludged together receiving loop antenna on the south facing wall of the room, my radios are set up in.  This reduced my BCI issues fair amount (but not completely - I don't know if they ever could be, short of moving to a new QTH), but I got sick of seeing a wire draped along a wall, to the room's doorjamb.  But, I've decided that it might be worth giving an indoor receiving loop a try again - only this time with its size being smaller, and with it being mounted on a steerable frame (to hopefully get a better null on WSSP).  

Would any of you have any ideas for steerable indoor receiving loops (loop diameter, number of turns, etc.)?  I'll probably end up having to experiment with the parameters just mentioned, to determine what performance trade offs vs size I can live with, but any info I can get, to eliminate starting from ground zero, would be great.

Thanks,
Ellen - AF9J

A quick loop would be to wind an self supporting 18" diameter loop of 20-30 turns of wire (I am guessing at the turns count) and resonate the loop with a variable cap to the band of interest. Then wind another single loop taped to the large loop and use this as a pick up loop to the radio. You can tape the whole thing together and mount it so it can be rotated. - The larger the loop area - the more signal - same with the turns count  - but the more turns  - the lower the self resonance frequency - due to the winding capacitance.

This would be a pretty cheap start.

Pat

I would suggest putting a small preamp/impedance match circuit at the loop so that you can attach a reasonable length of coax and move it to a position where reception is best and local noise is limited.  Otherwise the shunt capacitance of the coax combined with the very low antenna impedance will create a lot of attenuation.  This is why the whip auto antennas used special low capacitance coax to connect to the radio.

If you could place the loop outside on a patio (or even outside of your window) it might help you to avoid noise from the AC wiring. 

Here is a link to info on the Drake AL-4, although designed for BCB use it will provide a reasonable design: http://www.dproducts.be/drake_museum/al-4_loop_antenna.htm

There is a  good one described in the mid-70s ARRL Antenna Handbooks. It consists of a one turn loop, about 5' square mounted on a rotatable wooden frame, using RG-59 and a tuning capacitor.  The one they describe is primarily for 160m, but I built mine years ago and it can be made to work on 80m as well.  Probably on 40m too, if scaled down to reduced size.

The null is very sharp, and might eliminate your local BC problems, as long as the primary signal is not being re-radiated by other near-by objects. On 160m I usually get better reception with the loop than with the full size vertical, but still do better with the beverage in its intended directions.  The advantage I found (have had no local BC interference problems) is improved S/N ratio. When my shack was in the house, I didn't even bother to make it rotatable; just hung it on the wall like a large wall map.

In the winter I have an intermittent noise that shows up on 80 meters.  I first tried the coax loop but I ran into issues with weak signal reception.  I am in a rural location and the base noise floor on 80 is quite low.  I then used the same frame but used a 4 turn loop with the same preamp at the feed point and it works very well for reducing the noise source while still allowing very weak signals to be copied.  But since you probably have a noisier location the coax loop may be the best bet for you.

Do some google research on loops. I think you will quickly find that all the hoopla on using shielded loops vs unshielded loops to reduce noise gets shot down pretty quickly. The advantage for any loop is that you can null out the noise. The shielded design does not "shield out" or filter out the noise. You likely will need a preamp but for starters the one in your rig may work.

I have been doing quite a lot of research on loops and other small antennas for receiving lately so some of this data is fresh in my mind. W8JI has lots of loop data as well as many other websites. If I remember correctly - the ARRL loop was not considered a very good one.

http://www.w8ji.com/magnetic_receiving_loops.htm (http://www.w8ji.com/magnetic_receiving_loops.htm)

Pat

I looked at the W8JI page.  He seems to go on at lenth on what doesn't work.  People like me who do not have the luxury of hours to spend wading through these long treateses just want to get right to a design that is supposed to be good, build it and use it.  If I sit around reading, the winter operating season will be over by the time I have something.

From my reading, I think loops have a couple of potential advantages in dealing with noise. A balanced, shielded loop seems to be able to reject the electric field, and couple into the magnetic field of the signal. Most noise has a lot of electric field component, so rejecting that and only responding to the magnetic portion of an EM wave can give you 20-30 db noise reduction. That's the big selling point of loops like the wellbrook 1530 or Pixel RF-1B. They can help cut down on the part 15 non compliant hash that is beginning to surround most inhabited areas.

Loops also have a pattern with a deep null if they are constructed well, and that null can be aimed at a source of noise or interference. It doesn't help much in skywave propagated interfering signals, but can help a lot with direct or groundwave signals.

Ellen in your case, I don't think the shielded magnetic response is going to help because your problem signal is an EM signal with plenty of magnetic component. But the pattern null may help knock it down to a tolerable level because it is local to you and coming in direct path.So you may be fine with an unshielded loop, but make sure it's a well balanced physical design so it has a sharp, deep, null. Or a shielded loop may desirable to also help with the EMI/QRN sources.

The other thing I would experiment with would be a suck out trap. One of my friends has a 5 kw transmitter only a few hundred yards away and building a series notch filter and tuning it on to the AM station frequency helped a lot in his case. Or you might consider building some sort of high pass filter to pass say 80 meters and up. Clifton labs has some AM band notch or highpass filters as well.

Since you have a separate receiver, you don't have to build either of those kinds of filters very robust. It would be different if they were in the transceiver antenna path. You might even be able to fit the filter inside the receiver chassis to avoid having to build it in a shielded enclosure with coax to the RX.

I would investigate a series suck out trap in the receiver first I think. If that works, then you don't need to build any special antenna. If it helps, but not enough, then a loop with another 20 db of nulling might put you over the top.

Keep us posted on your progress.

As I stated - Do a google - There are hundreds of loop designs. I realise that some people can't stand W8JI  - but I find most of the time his theory is sound. He certainly isn't the only one who states that shielded loops are not "noise sieves"

The net is full of cookbook designs - Many are designed very well - but some are designed by idiots - pick the one that you like and build it.

Ferrite loops are proven and are small.  You need a big ferrite rod - longer and fatter the better.

A hula loop is a small plastic hula hoop with ten turns or so of wire wound around it with electrical tape holding the wire to the hula hoop. The coil is tuned to resonance with a varable cap. Another single turn is taped on and this is the feed to the radio.

I have always heard that noise is an mostly an electrical field. I have heard also that a magnetic loop is less noisy because is picks up the magnetic fields only - That is what I have always heard.

Unfortunalely - most of the reading that I have done in the last week or so say that this is lore and untrue.

All electromagnetic waves are composed of an E field and an H field at 90 degrees - so I am not sure how noise can be missing the H field component - Maxwell doesn't allow that to happen?

Am I missing something?

Pat

My first loop attempt was the ARRL design but I wasn't happy with it.  The current one uses 4 turns of regular hookup wire with a small trimmer cap across the feed point.  The cross arms are a pair of 4 foot pieces of PVC and it took maybe 30 minutes to build including the feed point mounted preamp.   

It feeds into one port of a MFJ-1026 noise canceller and the regular transmitting loop (post T/R switch) is fed into the other port.  I bought the MFJ noise canceller as my original attempt at removing the noise but it wasn't terribly successful although it did an amazing job in phasing out some interfering stations.  Now I just use it as a level control to choose between receive antenna sources.  Fortunately my annoying noise source, which shows up intermittently every winter, has been gone for 3 weeks now.  The loop does null it down to close to my normal noise level but the loop is outside and sometimes heavy wind gusts will move the loop enough to let a burst of noise through.

In addition to the harmonics you may be getting some interference from overload due to the broadcast stations and a high pass filter along with the loop may help.

Just found another interesting read at VK1OD.net explaining the shield and what it does in a shielded loop. The gap causes the signal in the shield to couple to the inner conductor and if properly place the shield induces more balance in the loop and deepens the null.

www.VK1OD.net/antenna/shieldedloop (http://www.VK1OD.net/antenna/shieldedloop)

Copied from the article

The gap in the shield provides a means of coupling current on the outer of the outer conductor to the transmission line formed by the outer surface of the inner conductor and the inner surface of the outer conductor. The inner conductor is not directly subject to incident EM wave because it is effectively shielded by the outer of the outer conductor.

The source of the current flowing on the inner conductor is entirely the current flowing on the outer of the outer conductor at the gap, coupled through the gap as described above.

The antenna is no more or less subject to the influence of electric and magnetic field components than an equivalent loop with the load connected directly at the gap.

The advantage of this feed arrangement is that the coaxial feed typically enters the loop opposite to the gap, and if attention is paid to symmetry of the loop and feed, the balance that is achieved. Best balance yields the deepest null which is important in direction finding applications for instance.

Notions that the shielded loop is not sensitive to E-field by virtue of an electrostatic shield formed of the outer conductor are not based on fact.

quote ends


I also agree that a trap and high pass filters are a good idea. Likely the station is overloading your receiver front end and much of the garbage may be generated in your receiver front end.

Pat

He basically states that the popular explanation of the shielded magnetic loop is wrong, that the coax loop is but a variation of the classic unshielded loop, and that the  coax shield doesn't electrostatically shield the 1-turn inner conductor loop.  I would have to re-read the article to remember all the details, but as I recall, his disagreement regarded more the theory of operation, than how well the loop actually does or does not work.

If you search the technical data  - he is just one of many that debunk that the magnetic shield blocks the E field.

I never understood how noise is E field mostly - doesn't agree with Maxwells equations.

All electromagnetic waves from VLF to light are composed of both at right angles to each other.

Pat

It's a near field/far field thing. In the near field, the e-field may be dominant for many noise sources. But if you have noise sources that close, I wonder just how much any any antenna will be able to null it out.

If BC fundamentals and harmonics are a problem, I'd deal with them separately. There was another thread here recently on wavetraps and highpass filters. Once those problems are resolved, go after the noise.

Good point on the near/far field issue. If you look at some of the videos for the pixel technologies loop, they show a signal pretty much buried in a frying noise like you might get from a nearby motor. When they switch to the loop, the signal to noise ratio against that noise improves dramatically. I'm pretty sure I've seen similar video for the Wellbrook loop.

I've recently acquired a Pixel RF-1B, but am just in the process of getting it set up and working so I don't have much experience with it yet. I got it as a dedicated antenna for my SDR receiver and RX2 input for the flex. I'm looking to combat a really nasty noise source that shows up from my neighbors house in the winter. (sounds like some sort of 60hz contact arcing and energizes the power lines for a quarter mile) I'll report more about that when I have some experience.

I don't think the E vs H field is too germane for Ellen's issue though.  She's far enough from the BC transmitter that she should be in the far field, so the potential E field rejection is pretty moot, although it may help with other noise sources that are closer.

What a loop may do for her is allow her to put the null on the BC station and reduce it's contribution in the receiver front end by 20 db or more. I got the rotor on the Pixel last night and spun it around and got 20-30 db null when it's pointed right for my local BC station. The construction of the loop (and as noted previously perhaps the shielding) will have a lot to do with how deep and sharp that null is. But knocking down the input of that station into the receiver will help and may be enough to get it to behave better, perhaps enough that nothing more is needed.

I think filtering with a series suck out trap to ground at the BC frequency (or multiple frequencies with multiple traps if needed) may also be worthwhile and pretty easy to construct, especially if you can lay your hands on some old loopsticks. 

Either may be sufficient by itself, or both combined may be needed.

Also, Ellen, you may want to experiment with grounding. At my friend's station he made some grounding changes that pretty much eliminated the problem on most of his rigs.

Yep - I'd first try a high pass filter of good quality - next I'd make a trap tuned to the interferring station - then if that didn't do it, go for a loop to try to null out what's left.


I have made excellent and very sharp bandpass filters with a three gang 365pF variable cap. Each of the three sections are paralleled with a toroid inductor and coupled with a small cap - 10 pF or less. Each stage is set to resonate about mid range of the cap. Set the filter on the problem station and peak the response for each stage with the mica trimmer on the variable cap.

You could put this in the antenna circuit wired to ground as a trap and it should really knock down the
offending station or you could substitue mica trimmers for the three gang cap and individually tune each stage to the station if you only have one station that is the problem.




Pat

Wow!  Lots 'o suggestions!   :)  Thanks guys!   BTW, the HQ-129X has a built-in antenna coupling capacitor, AND it's antenna feed is twin wire (I assume balanced).  I'd prefer using a pre-amp if I could.  I wonder, would a hybrid  (say one with 8 or 9 turns, instead of only 3 or 4), possibly help receive sensitivity.  I might try one this weekend, as a test mule of sorts. The high pass filter also might be worth a try.

73,
Ellen - AF9J

Hi Ellen
We recently had a nice thread about blocking out BC stations, and I'll be flipped that I cannot find it!!!!!!
There is success!!! The loop will not help much. You are too close.
Can someone find Clark's thread on his BC overload problem??? Pls

fred

I have a mid-70s ARRL Antenna Book and I was going to look at it last night but the Jan. ER arrived and I got absorbed in that instead  ;D  I will make a note to see what it says about loops tonight. 

I have also been considering the idea of dual ferrite loopstick antennas for 160, separated by 20 or 30 feet. 

Ellen, a loopstick has a pretty sharp deep null.  I have no idea if they work on HF or not though.  I am only concerned with 160 right now.

The search feature in the software that runs this board sucks.

Ellen ; I have had very good success using a Broadcast bandpass filter..Clifton Laboratories make some that still give 160m use.
I have used PAR as well as I.C.E.. The later is no longer available. Universal has PAR and Clifton is on-line,super quality..  73, lane

Geez fellas. I thought hams were supposed to be technically competent. Using the search function yielded the topic below as item number 1.


http://amfone.net/Amforum/index.php?topic=29929.0

Ellen,

The last thing that you need now is to add a preamplifier  - The station is already likely overloading you receiver and generating most of the crud inside your receiver. You need to filter out the loud station first and formost.

Pat

They're still pondering VR tube in DX-60B.  ;D

Sorry Steve, but Don is correct.  the search engine that comes with the bboard software sucks.  More precisely, it is crude and limited.

The solution is to go to google and enter search arguments for amfone.net.

the syntax is for example:

site:amfone.net bc band filter

google search rules apply.  if you want an exact phrase, enter   site:amfone.net "bc band filter"

I don't see the problem. I click on Search icon: http://amfone.net/Amforum/index.php?action=search
Put in bc band filter
The thread in question was 1st on the list although there many threads listed because it found anything with "bc", anything with "band", anything with "filter", and anything with any combination of those words in the phrase.
If you want to tighten the list, use quotes around your phrase: i.e. "bc band filter" as shown in the sample. (Same thing you did with Google.) Only four threads came up. If you use Google and the same quoted phrase, you get 1,110 results. You do the math.
You can also use the Advanced Search to further limit how it searches.

OK that's the one!!!!
Ellen you can figure out your problem with that thread.
Steve, I typed KE7TRP to begin my search using AMFONE and it never got me to the thread. It showed many other posts by Clark, but not the "BC band filter" title
Clark is a happy camper now.
Fred

LOL. You guys need to learn how to use the search before P&Ming.   :P

The screen capture below is the output from the Advanced Search on this forum, using the keywords bc band filter and the time as the last 30 days. Only two threads come up - this one and the one we were looking for - and it was the top one. It doesn't get any easier than this.

Some other receiving loops discussions we've had here.

http://amfone.net/Amforum/index.php?topic=12490.0

http://amfone.net/Amforum/index.php?topic=19022.0

http://amfone.net/Amforum/index.php?topic=27624.0

http://amfone.net/Amforum/index.php?topic=28994.25

LOL you need to read my post better you omitted site:amfone.net
  I stand corrected that was the search result.  However, so what--the sought after hit was the first one.  And, many of the other hits are useful.

I see the "advanced search" page or whatever you call it.  How about having a link to it that actually says "advanced search" instead of a dinky meaningless hourglass?  

Where in your search instructions does it say to use quotes for exact phrase searching?  I might correct my previous comment--your search engine is somewhat primitive; your instructions suck.

Wow. You're on a roll tonight Rob.

See the link that says Advanced Search?

http://amfone.net/Amforum/index.php?action=search

No Steve YOU are the one on a roll--go to the main amfone.net page--the one everyone sees and uses...see the absence of anything that says "advanced search?"   I only see the hieroglyphic and meaningless (to the literate) tiny magnifying glass.  OTOH, if you want to insure your search engine isn't used satisfactorily....

The point is that if you have an advanced search and you want people to use it, it does no good to bury it--there should be an "advanced search" clearly placed in a prominent location on the top by the search box that people can find and use.

Rob, the forum software is what it is, and doesn't lend itself well to customization.  I believe vBulletin is roughly the same (depending upon how you set it up).  There are many forums around that don't have working search functions at all (Antique Radio Forum being one).  For those you need to use Google (site:theforumyouaresearching.com).

Regardless, if you just clikc on the "Search" button with nothing in the search box, the "advanced search" link comes right up.

Whatever toots your horn. I think you should continue to use Google if it works for you.


You can't "Advance Search" until you get into the "Search" section after you click the SEARCH icon on the home page. Not everyone requires/desires the need for an "Advance Search"


Right below the Search Field: e.g. Orwell "Animal Farm" -movie
You obviously knew about the quote function, since in your previous thread, you use it in Google: You wrote:google search rules apply.  if you want an exact phrase, enter   site:amfone.net "bc band filter" but since we're already on amfone.net, one only has to enter "bc band filter" in the search field.

You're right Steve.  Hams are SUPPOSED to be technically compentent!

Okay yes thanks.


This dispute if you will, came up because someone was looking for a specific item; not just topic mining.  For a specific item, you usually need methods that allow some precision in searching, such as found in advanced search.  



You still don't get it.  What you say is true, but it is not immediately obvious that this way of searching is possible.
usually word set searches, which have default conditions like boolean or, get a hodgepodge of results, especially if the words are common AM terms and the user has to page through lists of results looking.    Most users probably just put terms in the search box on the main page, fail to find what they are looking for (or get lucky and find it), conclude that's the extent of the search function and quit.  

I'm simply trying to point out that the current user interface is to me, not all that clear.   If it can't be modified, okay whatever.  This isn't worth having a pissing match over.  

p.s. one nice thing about searching this site via google is that you don't have to login to search.

Well, while we are bitching about searching there are times google annoys the hell out of me:

Issue 1:  It always wants to correct you.  For example I recently spent a lot of time looking for some Eldico gear but google always defaults to searching for Eldeco instead and then asks, did you want to search for Eldico instead?  I guess google knows I am from Mississippi and assumes I don't know how to spell :(

Issue 2:  Even when searching using literals, the search engine also selects close matches and when the close match exceeds what you actually want by a 1,000 to 1 ratio it is just a tad annoying.

I don't use the search function that much on this site but I do use it some and it has never managed to raise my blood pressure.

OK, now I will have some coffee and turn into my usual sweet and loveable self.

FWIW, I also use Dog Pile every now and then. I don't think that it will put Guggel  out of business, but a little competition helps things along.  Whatt wood the whirl look like if tha hole whirl was Windoze??


Also,  Gary, INR, had a indoor loop that werked ok.

klc


Hating Microsoft since windoze 3.0

For years I've been using the loop Don describes scaled for 75. It is a tuned loop, tuned for 75 and is pretty deaf elsewhere which is a real plus on some occasions. I built it out of a much later ARRL antenna book so you shouldn't have to scrounge around for the '70's edition. The author is Doug DeMaw.

I know there is a controversy about just how these things work but I can attest that they certainly do work especially for nulling out a local noise source.  

HEYY Ellen,
So how's your head doing???
 I'll have to stick to what Clark had with his overload and images problem and highly suggest a BC band filter to reduce by many dBs your broadcast channels
Keep us informed.
Good luck
Fred

Of course everybody knows that the classic indoor antenna was the bedspring and o course the loaded bedspring array.

Or if there was one bedroom upstairs and another one on the storey directly below, you could use the lower set of bedsprings for the counterpoise and the upper one for the flat-top, and run the antenna wire up the wall and through the floor, hoping no-one would notice the tiny hole you drilled in the floor.  ;D

Ummmm, that turned out to be a typo on my part.  I just noticed it like 2 minutes ago.  Oops!   :-[  I actually meant to say that I don't want to use a pre-amp.   That's what I get for not proofreading my post!

The only case where I suggest a preamp is if you have the loop some distance from your receiver.  It will behave mostly as an impedance match between your very low impedance loop and the coax, otherwise attenuation will be very high from the shunt capacity effect of the cable.  The alternative would be to use low capacity coax (like the old automotive type) if you can still find it and I have a feeling the shielding ability of it is poor.  As I recall, the low capacity was achieved by using a spiral wire as the shield.

Neither the coax loop nor my 4 turn loop worked well feeding 25 feet of coax directly but the addition of the homebrew preamp right at the loop woke it up nicely.  You would likely need a shunt trap right at the loop connection point which is  tuned for the most offensive BC station if you need to use a preamp.

Thanks for the links to the filter threads guys!  :)  In a nutshell, first and foremost, I intend to try a 4 or 5 turn loop for tomorrow morning's 75m AM swap net.  If that doesn't cut it, then it will be filter time.  I think the HQ-129X will be a nice match for my Viking II.  

Lots of ideas in this thread. loops are fun to build and give a great solution to noise.

I have used preamps on broadband single turn coax loops with limited gain to get them up to the RX level of a normal dipole mostly for A-B switching convenience that gaining much in terms of signal to noise, and yes, these are usually outdoor types and they are pretty big. That said, I will take a bad beverage over a good loop.

Indoors I would use a high Q tuned loop with good balance and Faraday shielding - on the main loop as well as on the feed loop if it uses one.  Mike WU2D