HF noise.

[mickm3for]

Hi the hf noise is around s8 across all bands so i decided to try and improve this
I looked at possible causes and i noted if i ran a 6 mtr lead directly to the elevated multiband vertical AE and a battery sdr radio the noise was a lot lower around s4 i concluded from this it had to be either coming in from the mains or the coax. 1 i ran a new length of rg213 no change, 2 constructed a comprehensive mains filter both common mode and diff mode no change 3 put common mode filters at both ends of the coax one close to AE the other close to the rig now qrm at s4 across the 80 to 10 mtr bands, the noise had to be picked up on the outer of the coax this was earthed. i had no rf in the shack and the match was good 1.1/1 the clue now thinking about it if i changed the length of the patch lead the swr would change, now no different what ever the length . Mick

[Bazz4CQJ]

What you need is a Magloop; build one for <£25?

B

[G6Tanuki]

I recently fitted a choke-balun to the feedpoint of my temporary [and rather poorly-elevated] 28MHz half-wave dipole; it didn't seem to reduce the noise-level but clearly did 'something' to reduce imbalance as the SWR dropped from 1.5:1 to 1.2:1

[I really must do something about getting a proper 28MHz antenna setup]

[FrankB]

It could be caused by SMPS in computer supplies or florescent lights with the transistorised ballasts.
Also I have seen it caused by mercury vapor street light ballasts.

There is some possibility of a cracked insulator on a nearby power pole, or a bad power co. ground. I had a bad power co. ground 1/2 mile from me causing a similar problem.
To find a bad insulator, take an AM radio with you & tune to the most noise. Whack the wooden power pole with a 5 lb or bigger sledge hammer. If noise changes, that pole has the problem. My neighbors thought I was crazy, but I did find the problem & the power co came and repaired it.

[Craig Sawyers]

Originally Posted by Bazz4CQJ

What you need is a Magloop; build one for <£25?

B

Yup. A long wire just returned wideband noise all the way to 30MHz. No sign of a radio station.

Bought a Wellbrook mag loop - total silence apart from radio stations. A real revelation.

Now Wellbrook are closing down due to the guy retiring. But you can build your own - there are recipes and an amplifier design that is better than Wellbrooks https://www.george-smart.co.uk/projects/wellgood-loop/ .

If mine packs up, I'll definitely build one of those.

Craig

[G6Tanuki]

Magnetic loops are all very well, but if as the OP indicates he needs to use it for transmitting, the Wellbrook and clones are not going to be usable.

I've got all the 'bits' [lots of lengths of 22mm copper pipe, 135-degree elbows etc] to solder together a decent low-band magnetic-loop antenna; biggest problem is that once you start to get into the tens/hundreds-of-watts region you need some significant RF engineering for the tuning-capacitor, and the 'Q' should be high enough that remote tuning [a second problem] becomes necessary. Manual tuning is a no-no because the presence of the person-doing-the-tuning adds stray capacitance which is removed once you walk away...

[Bazz4CQJ]

Receive on the magloop and transmit on the long wire / dipole etc.

I modified my TS530 so there's a separate "receive only antenna socket" which eliminates the possibility of sending the total amount of RF through the magloop.

As to the question of how resilient a magloop is to stray RF from the transmitting aerial, I guess that depends on several factors.

B

[G6Tanuki]

Originally Posted by Bazz4CQJ

As to the question of how resilient a magloop is to stray RF from the transmitting aerial, I guess that depends on several factors.

B

That would be my great worry about using a Welbrook-type amplified loop in the local presence of a TX.

There's enough 'stray RF' flying around here when I run 100W on 80M to cause my broadband-router to lock up [overhead-wire phone lines are not helpful] and in times-past I remember repeated failures of a short 'active antenna' when used close to a transmitter.

There's also the issue that your receive-loop may be directional whereas the longwire/dipole you're using for transmission is differently-directional, so replies to your "is this frequency in use?" calls may not be heard and you start calling CQ over the top of existing contacts.

[Bazz4CQJ]

Yes, the price of "escaping" the digital hash are the various issues you describe. However, while some buyers of Wellbrooks were just listeners, I'm sure that some were transmitting alongside.

Of course you can rotate a loop with very little effort. There's one member of the forum using an model aircraft servo mechanism to do it, and as soon as you have a steerable loop, then there's another tick in a box.

I don't think anyone is claiming that the magloops are the solutions to all problems, but they are an interesting option when trying to tackle noise on the SW bands.

B

[G4_Pete]

I use 2 loops at 90 Degrees and can switch between them which covers the directivity nulls quite well and is great for all band low noise RX.
BUT when I split the RX/TX on my IC7300, easily done via available mod kits using plugs just inside the cover, I found the TX side ALC and SWR indication of the IC7300 is sensitive to RF coming in on the separate receive socket giving incorrect SWR readings and TX instability.
I measured the RF coming in from the active antenna and it was in the volts range with about 25 Watt TX on the doublet TX antenna so put back to standard and having a re think on antenna separation and switching out the active antenna.

Not sure how that is handled in the transceiver normally but is a big problem with a separate loop.
Pete