[RFI] High Noise Levels on 80m

Tom Rauch w8ji at contesting.com
Tue Dec 19 22:21:13 EST 2006

> In a 500Hz CW bandwidth, I typically have about S0 or S1 
> noise on all
> bands except 80m.  Sometimes noise jumps up to as high as 
> S6 on other
> bands, but most of the time it is pretty low.  80m, on the 
> other hand,
> is S7 to S8 24/7/365.

Several factors enter the equation on 80 (and especially 
160). Ground wave goes much further, antennas are closer to 
nearfield noise sources in terms of wavelength or antenna 
dimensions, and noise is conducted along wiring much 

By the way ground wave can only be vertically polarized on 
low frequencies. This is because the earth shorts any 
horizontal electric field, and when the time-varying 
electric field is taken to zero so is the magnetic field. 
Thus the only thing that propagates along the earth any 
distance is vertically polarized. Keep this in mind.

> Question 1 - I know that 80m has more noise than 40m, but 
> should it
> really be 6 or 7 S-Units higher than 40m?  That seems like 
> a lot.

Well, the antenna even if at the same height is twice as 
close to other things in terms of wavelength. Worse yet, you 
have the ends near ground.

Now all dipoles respond to vertically tilted waves to a very 
noticable extent, and that response is maximum off the areas 
of the end. It only goes perfectly horizontal broadside to 
the antenna. The Inverted Vee dipole, while perfectly 
horizontal in polarization broadside to the ideal antenna 
like any dipole, has increasing vertical response as we move 
around towards the ends. The Inverted Vee dipole actually 
has enhanced vertical propagation off the ends.

So you have three things against the antenna being quiet:

1.) The frequency is lower so the antenna is electrically 
closer to any noise source
2.) The ends are dropped very low, so the antenna has 
enhanced vertical response in directions near the end 
3.) The antenna itself is probably closer to noise sources

Now there is one more issue. Many people do not use good 
baluns on dipoles and so the feedline and everything 
connected to the feedline is part of the actual radiating 
system. This means the radio chassis, the power line 
connected to the radio chassis, and everything generating 
noise on that power line has a direct wire connection to the 
antenna. This can be a major noise contributor in today's RF 
polluted world.

> Question 2 - Normally, the noise level in a 2.4KHz SSB 
> bandwidth is
> considerably higher than in a 500Hz or 300Hz CW bandwidth. 
> I can see
> the S-Meter drop several S Units when I switch between 
> wide and narrow
> bandwidths.  I do NOT see this same drop on 80m.  There is 
> no drop
> when I switch bandwidths or if there IS a drop then it is 
> less than 1
> S-Unit.  This doesn't make sense.  Can anyone explain to 
> me why this
> would happen?

If the noise is broad spectrum random non-coherent noise the 
detected noise power is directly tied to filter bandwidth. 
Half the bandwidth is half the noise power. Until you get 
down to the BW of the signal S/N will keep getting better.

If you are not seeing that relationship and the receiver 
bandwidth is really narrowing, it could be your noise has a 
phase relationship that varies with frequency. Noise a few 
kHz away might not add, it might even subtract. It could be 
your 80M noise has a very high peak to average power ratio 
and when you narrow the filters the noise bangs the filters 
or some other circuit so hard it distorts or rings, keeping 
the average level up. DSP radios with all the selectivity 
following ten stages and a none-too-costly A/D converter 
trying to sort out all those rapidly changing voltage levels 
without overflowing like a toilet in Tijuana can be (and 
often are) a problem. You might try using as much 
attenuation as you can in case very high sharp peaks on the 
noise (compared to averaged power of the noise) are 
overloading or overdriving something in the receiver.

73 Tom 

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