The AM Forum

Has anyone found that a standard folded dipole is lower noise receive than a standard dipole?
Thanks..........Larry

Only if it has more loss.

In theory a closed loop antenna is suppose to pick up less noise,  not sure if this follows to a folded dipole.  I would think it does but you would have to do an A-B test to really tell.

Fred

I compared a 40M dipole to a Double Bazooka. The Bazooka was abt 1 s-unit lower with local QRM like ignition noise and power line hash. Station signal strength was the same. The Bazooka is a somewhat shielded design.

Hi Larry,

To find out, the two antennas would have to be placed in the exact same spot to accurately measure the effects of local noise. This would be hard to do.  They would couple and produce similar results if too close to one another or if measured separately, would make time and varying band conditions a factor..

My question is, how does an antenna know the difference between a desired signal and noise? How would it know to suppress the noise and let through the desired signal? It doesn't.    

Sure, if transmitting 500 Kw, the closed loop will have less corona on the ends than a dipole, but during receive, I doubt it.  

I have compared wire quad loops and folded dipoles against standard dipoles and could never see any difference in "noise" suppression. Again, the biggest determinant is the physical proximity to the noise source.  Even 100' can make a big difference and orientation to the noise even more. So two antennas in A/B comparison may be an inaccurate   test.

I have tried quad loops in the clear compared to dipoles in the clear separated by several wavelengths and could see no difference in snow static, atmospheric noise or any other unwanted power line noises.  From my own experience, I believe noise suppression due to a closed loop is an old wives' tale.

What DOES work well for noise suppression, or any signal for that matter, is phasing two antennas together to reject the unwanted noise source. The MFJ phasing box is an easy solution.

T

How about in the case of a shielded magnetic loop antenna. I use my Pixel loop on the AM broadcast band or 160 and I'm able to receive far more stations (especially during daytime) than any wire antenna I have. My local daytime S8 noise drops to S3 and the signals stand out. The loop is definitely better at suppressing local QRM.

Could it be polarization or directivity that the rotatable shielded loop exhibits?   As a result, the ability to null local noise and peak desired signals.

My 75M  800' Beverage that is 6' high shows a similar thing... a better signal to noise ratio, even though the desired signal in Europe is down 20 dB from the high phased quad loops.  The Bev has a very sharp uni-directional pattern. This allows yuge noise rejection off the sides and rear, thus the better S/N ratio. All about directivity.

T

Ok, next question. Will a resonant antenna pick up less local noise, or resonant and balanced, of course isolated from the feedline.
Larry

The time a closed loop picks up less noise is when you get an arc across your feedline :-)

Closed loop = DC short.   

Same as putting a static bleed on the antenna / rx input.

--Shane
KD6VXI

If your loop was shielded, it would pick up NO signals.

VE7SL has done good work in writing up his various loops.   The loops that look shielded actually are electrically broken, usually at the top to keep it from being a shorted turn.  They are not a continuous conductor, otherwise as has been previously stated, they would not work!  

http://members.shaw.ca/ve7sl/

Joe-GMS

If you go up on YouTube and search for the Pixel Loop you can find many examples. It's definitely not snake oil. The first year I got mine I worked 9 new countries on 160, using it as my receive antenna. For sky wave, it's not that directional so it's not due to directivity. My local noise on 160 in the evening is about S7 using either my inverted L or a sloper. With the loop it drops to ~S3-S4 and the weaker stations that are otherwise hidden by local QRM are now readable. During the day I can reliably hear AM broadcast stations 200-300 miles away using the loop. Without the loop, nothing but noise. Regardless of why it works, the end result is less local QRM.

Check these
https://www.youtube.com/watch?v=T8N_Ex9w2ac
https://www.youtube.com/watch?v=V7HPkcF75uc

I have the Welbrook one from Europe and have been extremely pleased with it.  Lots of low noise gain and nulls out local noise very well so the resulting SN ratio is a major improvement. 

Joe-GMS   

Steve, you raise an interesting point ... I believe electrically shielded is correct .... I think that these loop antennas respond to the magnetic portion of the em wave

we are in danger of being accused of hijacking this folded dipole thread, unless it is related enuff to an evermore common problem most hams face ...noisy rx

I am waiting till I know what kind of noise the originator was speaking about.

Shielded is shielded. There is no reason why the mag portion of a far field EM wave will pass though a shield at HF. None.

These loops may improve the balance to ground. This, combined with the directional nulls cans reduce some of the more local (i.e. ground wave) noise. So, yes, they can improve the receive SNR compared to a dipole. But it's not because they are shielded.

I'm all for hams building or buying and installing receive antennas. It can make the on-the-air experience far more enjoyable.

Good info at the links.

http://www.n6rk.com/loopantennas/pacificon.pdf

http://www.avionics.com/books/books-sample-copy/antenna-engineering-handbook.pdf

My original posting was specifically about whether or not a folded dipole was better at local noise reduction compared with a "standard" wire dipole. Though actually I did not say local noise. There were no replies that answered the question, but the whole topic of reducing local noise is worth talking about, so go ahead and hijack away!
Larry

To answer your question Larry, I don't see any reason why a folded dipole would receive any less noise (local or otherwise) than a single wire dipole.

A folded dipole made of twin lead or insulated wire will be less subject to static electricity generated by wind, dust or precipitation.  It will not help with man made noise over a dipole.

Good points. I agree that, except for local static discharge right on the antenna, a folded dipole should receive signals and noise exactly the same as a standard dipole.

Here's my theory on low-noise receiving antennas.

I think one reason (but, don't throw rocks, it may not be the only reason) that small loops and beverages have a better signal to noise ratio than our main station antennas is simply because they are mounted close to the ground.

This helps the S/N ratio, I think, because most local noises are not, as popularly believed, received as ground waves but as direct waves. The random hunks of power wiring and interconnect cables that serve as the noise sources' "transmitting antennas" are not efficient low angle radiators as a rule. They are essentially low random wires, which radiate mainly at high angles.

These noise sources' radiation essentially sprays upward toward our nearby high wire antennas (whether our antennas are horizontal or vertical) and hits them full force. These noise sources radiate relatively little along the surface of the Earth. So low receiving antennas get a lot less noise. There is a semi-quiet zone near the ground.

This explains why many different kinds of low receiving antennas, including dipoles three feet off the ground and random pieces of wire laid right on the ground, can often give us better S/N ratios on receive than our main station antennas.

Great info, thanks for posting.  Have been considering a receive loop as I'm on the edge of a community and the noise can get pretty high. Apologies for the highjack.

73s  Nigel

thanks for posting, Steve .... there is a lot of gce (gross conceptual error) including mine about loops .... I feel better knowing Terman made a mistake too .... he didn't make many

This has been a very interesting thread.  Thanks guys for the info.  I'm seriously considering some sort of magnetic loop solution to my occasional noise problem.  I have another need where I want to notch out a mid-western station who comes on top of an existing QSO and tries to bully us off frequency insisting that his net has to take precedence.

Notching him into oblivion would be an excellent "tool" for HF operation

Thanks, Al

It's unlikely that you will be able to notch out any interfering station with a loop. The loops are largely omnidirectional on skywave signals.

I agree with all of the above on skywave noise. But local noise is the bigger issue.

I use a big square vertical loop (250 ft total) for receive, bottom fed with a 1:12 Balun and top resistively loaded. It is at 75 feet on the top horizontal and over your head at the feed. It is very effective. Signals I can not make out at all on the 130 ft Inverted L, easily come in over the somewhat bad noise floor I have. An A B comparison on strong signals proves that the inverted L has somewhat more gain, however on 80 and 160M.

I agree that a dipole would give similar results as a loop in the woods, but in an urban or relatively noisy suburban area with lots of electronics around, I want a loop or a beverage.

Will a folded dipole would give some added noise rejection over an ordinary dipole, both being resonated to 1 band and properly choked? I have never tested this beyond a Bazooka (as mentioned before and it was quiet), so I cant say for sure. But in my head, the ordinary dipole looks like a voltage probe at all frequencies below resonance where most of the local noise lives and it still picks up pretty well. I have to think that the folded dipole antenna would give added noise reduction, especially out of band, to manmade noise, but I am probably wrong.

It's good to learn new things.

There may be a reason why a folded dipole could receive less noise than a single-wire dipole. It has to do with balance and the ingress of noise (probably mostly local noise) via common-mode on the feedline (in the case of coax feed). I came across the link below, wherein there is some information on the self-balancing nature of folded dipoles. The self-balancing minimizes common-mode feedline radiation, even in the case of imbalance in the feedline/system. Less feedline radiation, and possibly the antenna balance itself, could result in less receive noise, in some cases.


http://www.hamradio.me/antennas/loop-fed-array-lfa-planar-yagi-uda.html

 If the folded dipole fits in the backyard and the classic dipole needs to be strung over a house, the folded dipole probably will be quieter. Experimentation is key.