Baseline hiss/noise pick up on antennas

[RadioDaze2.0]

I thought I would start a discussion about hiss. Assuming an electrically quiet location away from manmade sources of electromagnetic interference I have noticed significant difference in noise pick between my Gain Master and my IMAX2000 (no radials)

When I use the IMAX2000 I notice the noise floor is higher by some margin. I still have a S0 noise floor like the Gain master but I find the hiss level is much louder on the IMAX meaning that I do not need to have the volume up as high as the Gain Master to experience the same noise level.

I imagine that RX hiss is simply listening in to the background radiation of energy from in essence the universe. I imagine much born from stars and endless decay of electromagnetic ejection of "space bodies"

So I am trying to understand if simply the IMAX2000 is a more efficient receiver of all signals or just noise.

I find something in this article of interest and have never seen anything written about this before.

http://www.copperelectronics.com/discus4/messages/7750/20506.html?1023340740

"Since most man-made interference received during the day is sky-wave, and usually high angle sky-wave, an Imax 2000 with the GPK installed will likely experience the 'noise floor' reduced by as much as 12 dB. This is quite significant if you are trying to carry on local communications when sky-wave noise is very high."

After mentioning this perceived different in hiss RX between the GM and the IMAX2000 a French station that I spoke to suggested it was because the IMAX was a signal hoover and was very good at RX. I said I felt the GM was also a very good antenna (responsible for my most distant and impressive contacts) and seemed to have a quieter RX. It made me think could it be the IMAX has a much better RX or is it just a better transducer of some form of noise ?

I would like opinions and educated responses related to RX of noise of 2 antennas of comparable size. i.e. 5/8 wave long one being a ground plane type (or at least a basic radial less 5/8 wave) and the dipole like GM.

Could it be the IMAX does have a 6-10 dB (estimated by ear) better receive (highly doubtful) or does it just pick up this sky born man made noise or just general "cosmic" noise better than an IMAX. I cannot recall what some of the plots Marconi/The DB has made for the GM and IMAX and whether they relate to those in this web page with regards to this pick up of noise. I have seen some horrendous looking IMAX2000 plots with very high angle lobes when a bad mast and coax length is chosen which badly effects the pattern due to high CMC presence (though I understand it could not be replicated here by the 2 best know antenna modellers The DB and Marconi)

I hope this is of interest.

 

[tba02]

I'll go out on a limb as say the GM coil (choke) at the base acts as a line isolator and reduces the noise level (hiss) just a bit.

. just a wild ass guess at some level, but I have seen similar when adding an isolator to existing antenna setups (moxon).

 

[Robb]

Both have fiberglass radomes(?)
One is an end fed 5/8 wave (Imax), while the other is a vertical dipole 5/8 wave.
One requires radials (Imax) for max efficency, and the other does not (arguable).
They both use matching sections for impedance matching.
One uses an isolation coil (SGM) while the other doesn't.

 

[vkrules]

At my location the gainmaster always has a lower noise floor ( but not signal) than the imax.Also noticed that on the long haul contacts ( 16000 km, not this short across the Channel stuff) The gm also had slightly higher sigs.
Not sure why! but I do know the gm has less cmc issues.and my guess is it produces a cleaner pattern with less high angle radiation.
Interesting test your end would be to fit choke and ground planes on the Imax and do another comparison..

 

[RadioDaze2.0]

At my location the gainmaster always has a lower noise floor ( but not signal) than the imax.Also noticed that on the long haul contacts ( 16000 km, not this short across the Channel stuff) The gm also had slightly higher sigs.
Not sure why! but I do know the gm has less cmc issues.and my guess is it produces a cleaner pattern with less high angle radiation.
Interesting test your end would be to fit choke and ground planes on the Imax and do another comparison..

Thanks vkrules that is interesting. I suspect you might be right and my experiences on air have alluded to that. My longest DX has been to NZ long path using the GM (about 21,000KMS). That could have happened with the IMAX2000 but I am doubtful. Of course the RX and TX patterns are typically the same. (reciprocal)

I have found a ferite choke effects the SWR negatively and a coil or ground planes are impractical for my current methods of set up. Though it would be interesting to compare for sure.

I think the GM is an antenna with a more reliable/predictable results in various set ups. It seems less ambiguous to me than a 5/8 with no ground planes/isolation, CMC protection etc. It definitely seems to have some kind of noise pick up the GM does not suffer from. It has been very widely reported online as well in many installs.

I was getting a feeling that the IMAX2000 had a null at some angles although this could easily be deaf stations at the other end or simply one way skip.

Interesting to know peoples thought who have used both antennas.

 

[vkrules]

If the choke is affecting you SWR negatively. your coax is radiating and thats your noise right there..

 

[vkrules]

Just adding to the previous post.If your coax is radiating, for what ever reason,not only is the pattern degraded but the whole run of cable becomes a pickup for noise . This places noise sources like cars ,house hold appliances,power lines etc in the near field .
Without eliminating CMCs ,any comparisons is like apples and oranges.That's why I said try it yourself.
On another note the gm elements are completely encased in insulation and the imax had only about 2/3rds ( the top section being stainless steel) The gm also shows dc short across the coax and open circuit to the top radiator( being fed with a cap) these points alone might contribute to rx noise difference.

 

[RadioDaze2.0]

If the choke is affecting you SWR negatively. your coax is radiating and thats your noise right there..

Well I have heard this said before. My VSWR without any chokes is 1.3:1 at best. When I add a ferite isolator it goes up to 2.5:1 = unusable. So very obviously use it without.

There is no obvious reason for a high VSWR (i.e. no metallic objects around) 9M pole up with the IMAX on. The GM also reads a 1.2:1 SWR on the identical mount situation.

I am not categorically saying there is no CMC's on the Coax/Poles but neither am I clearly experiencing negative effects from any that are present. (other than the potential noise we are discussing) I am not going to use a choke because it makes the antenna unusable.

I wonder if the coil choke and or/ferite choke negatively interacts with the matching arrangement of the IMAX. Maybe it is a CMC on the feeder issue but in lieu of no known negative effects and no current clamp I cannot actually tell.

My radiation near field is empty of any noise sources as I am out in the country side. Only a car that is switched off. On occasion I hear crackling from spark plugs firing when a car goes past but I get that with every antenna I have ever installed. There must be a lot of EMF being produced and radiated. There is an old fashioned 1930's car that drives around and I can hear it coming from 300M away lol. Seems to have zero spark suppression.

"On another note the gm elements are completely encased in insulation and the imax had only about 2/3rds ( the top section being stainless steel) The gm also shows dc short across the coax and open circuit to the top radiator( being fed with a cap) these points alone might contribute to rx noise difference.

The top section on my IMAX is copper and the entire antenna is encased in fibreglass. In diagrams of the IMAX2000 I have seen a section referred to as a capacitor as well.

There are numerous holes to pick in this article but it seems like there is at least some kind of brass insert in the IMAX...

http://www.k3dav.com/solarconimax2000antenna.htm

To quote the relevant section:


"Near the inside middle of the center section, there is a high power capacitor-coupler (Shown below), and yes it radiates and couples the top half of the antenna. This allows the center section to radiate a larger pattern than most verticals do."

 

[Marconi]

RD, I've had mixed results using coaxial chokes.

Some worked to stop the RFI and some didn't.

Some worked for a few days and then seemed to stop.

Some effected the readings on my Autek VA1 analyzer, and I saw SWR changes that I figured were due to the added line length.

How is the choke added to your Imax?

Rolled up using part of the working feed line?

or

Is the choke rolled up using another length of coax with connectors, and then added to the antenna end of the working feed line?

 

[Tallman]

Two words can make a huge difference, Station Grounds.

 

[vkrules]

Yes Tallman .but same station same station ground.

 

[vkrules]

RD My Imax had a s/s top whip section. .Can't do any further testing with the imax sold it
Thought your testing might shed some light on the subject.
You use yours for portable use I believe on a f/glass pole.Maybe you need the first 9ft of cable as a counterpoise?
Anyway like I said if the choke is changing the swr( especially as bad as you state) You definitely have a problem with the coax radiating . The adverse affects are increased noise pickup and pattern degradation.
Now whats causing it, how bad it is and if you notice it or not are open to speculation.

 

[RadioDaze2.0]

Sure, well I use aluminium poles here and grounding on a temp station typically set up for 3 hours is not easy. And not allowed given I would be digging on someone else's land or a roadside and I change QTH as well). And it is at least not easy to do correctly. So it seems once again the GM is indeed an antenna that seems superior in terms of ease of set up and reliability in terms of lack of CMC presence on lines and poles due to proper decoupling.

Noise level becomes important with S-0 / R2-R5 signals which many of my RX signals are on the line of sight.

Pattern is also important and I smell a rat with the IMAX RX/TX lobes in my existing set up. I reckon on the very low and rather high angles being there with some significant drop off of signals between as a guess 30-60 degrees meaning some short DX might suffer performance issues. It's a hunch not evidence based.

Last year there was a nice short hop Sporadic E opening one late afternoon/ evening and with the Gain Master at 7.5M I called and got 2-4 stations come back from N.E.S.W. from each pole of Europe every CQ call. There was so much traffic I had to pick and choose the QSO's it was too much to cope with. Now that is what I call good conditions and an antenna that was working well.

 

[M0GVZ]

Two words can make a huge difference, Station Grounds.

The solution for the clueless or lazy. RF grounding must be done at the antenna, not the shack.You're still getting all the common mode coming down the coax and mixing with the wanted signal. Lightning protection must be done at the antenna and before the feeder enters the shack. The only station ground you need is an electrical safety ground.

Well I have heard this said before. My VSWR without any chokes is 1.3:1 at best. When I add a ferite isolator it goes up to 2.5:1 = unusable. So very obviously use it without.

Then there is something wrong with your installation. On my Imax 2000 I took it out of the box, never touched the tuning rings and just stuck it up in the air. 16ft pole or 21ft pole, RF choke or no RF choke the SWR stayed the same. The only thing that changed was a drop in TVI with a RF choke in place.

 

[Tallman]

The solution for the clueless or lazy. RF grounding must be done at the antenna, not the shack.You're still getting all the common mode coming down the coax and mixing with the wanted signal. Lightning protection must be done at the antenna and before the feeder enters the shack. The only station ground you need is an electrical safety ground.

Absolutely. When I say station grounds I start at the antenna. On a three sided mast that means three heavy duty grounds that start at the antenna and attach to three separate 8ft ground rods hammered in to the soil. Inside the shack you still need equipment grounds and not just house grounds either. Some house grounds are not all that great, and if you have GFCI breakers or outlets you can be tripping those.

Done to MIL-STD 1686