[RFI] high noise level...

[Ian White, G3SEK]

Tom Rauch wrote:
>Paul,
>
>I ALWAYS use a balun with any balanced directional antenna, because even
>small amounts of common mode can cause loss of directivity by filling in
>nulls. It just is a very good habit to get into, better safe than sorry.
>
Strongly agree! A choke balun at the feedpoint is the best and most 
basic precaution against RFI - either to your station or from it.

>Also ground the cable (if you easily can) at the mast top and bottom for
>lightning protection and reduced common mode problems. Otherwise tape it to
>the mast very well, and bring it all the way to the grounded base.
>

My guess would be that Paul has a crankup tower (fixed towers are rare 
over here) so he may not be able to follow the advice about taping the 
feedline to the tower. That makes grounding the shield at the base even 
more important.

>Understand this is all an educated guess, and real results can be
>exceptions, but:
>If the noise isn't terribly directive when you rotate the quad (assuming the
>quad is working OK and has good directivity), and doesn't change when you
>crank it up and down, then that is a strong indicator the noise is following
>the feedline or mast up to the area of the antenna. Either that or the noise
>is close to the antenna and in an area where distance doesn't change much
>when the antenna is raised and lowered.
>
Another possibility is that it might be coming up the rotator cable, and 
crossing over to the antenna at the top of the tower. Remember, there is 
actually no "ground" at the top of the tower - there are only "common" 
connections.

One way to check for noise currents on the antenna and/or rotator cables 
might be to wind any slack into choke coils at the base of the tower, or 
to clip on a fairly large number of ferrite beads. Neither of those 
might be a complete cure, but if the noise level goes down, you'll know 
you're on the right track.

Yet another way is to try transmitting, and use a clip-on RF current 
meter to check for RF on the cables. There should of course be none - 
but where your RF can get out, noise can get in.


>Did you try opening the breakers in your house one at a time, and seeing if
>something in your house is causing the noise?? That's always the first thing
>I do. Switching power supplies, lights, bad doorbell transformers, and
>dozens of other things can cause noise. Telephone lines can conduct noise,
>as can power lines. Arcs in power line also cause noise, and it can be
>conducted for long distances.

Good points, all well worth following-up. Even so, it seems suspicious 
that this noise should suddenly appear (and be so loud) when you've put 
up an antenna that ought if anything to be "quieter" than most.

Think *very* carefully about what other changes happened you made at the 
same time - there may be a clue in there.

Can you still check the noise level from your previous antenna, Paul? 
(Also what kind was it, and where was it in relation to the new quad?)


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek