G8JNJ

Questions About the Mini-Whip Antenna

G8JNJ

December 2019

Just a couple of additional thoughts following on from Stu's post.

Both techniques of screening and choking are important, but they perform different functions and have to be applied appropriately.

"… (more) Quad shielded" coax can be many things, but these days many of them are just marketing hype.

In the past coaxes with multiple screens that were insulated from each other were used for video distribution in TV studios, where it was necessary to separate 'clean' and 'dirty' electrical grounds from each other, so that very low level noise didn't get introduced into the analogue DC to <10MHz video circuits.

Quad shielded coax as referred to these days, tends to be a generic term that means it incorporates other cores around the inner, and some of these may simply be to provide additional mechanical strength or protection, such as steel reinforcing strands.

It's best to avoid coaxes that have a plated steel cored center conductor and also those with aluminium foil screen or aluminium conductors.

All of these can cause problems on the lower frequencies where the conductor skin depth is not adequate for the purpose on the LF bands.

Common mode noise is carried on the outside of the coax screen, and so as long as the skin depth of the conductor is sufficient, it will remain there and any additional screening won't make much of a difference.

The problem arises at the ends of the coax where it is terminated, and makes a transition from balanced to unbalanced due to the current on the outside of the outer screen.

Common mode chokes are required to reduce the amount of common mode current that can be carried along the outer of the screen and can be re-radiated to be received by the antenna. It can also be directly re-introduced into the balanced signal path of the coax by incorrect termination of the screen at that transition point.

The frustrating thing about common mode chokes is that it's a bit like playing 'Whack a Mole' as soon as you put a choke in one place, the current distribution changes and the noise can pop up on a different frequency. Some degree of experimentation is required to find the best placement, and this often changes when you connect new items of equipment, or even in some cases when you simply plug another item of electrical equipment such as an extension lead, and the long mains cable modifies the RF current distribution on various surrounding structures.

I have used an E-Probe mounted above the metal roof of an industrial building full of various items of consumer electronics and server farms. The trick is to use the metal roof (and frame of the building) as the ground plane for the antenna, but to ensure that the coax has lots of common mode chokes fitted on the 'building side' of the ground plane connection, so that noise from within the building is not introduced onto the antenna itself.

Regards,

Martin - G8JNJ (less)

Multiple "Sweepers" on my Cascade

G8JNJ

December 2019

It's Over The Horizon Radar (OTHR)

https://en.wikipedia.org/wiki/Over-the-horizon_radar

There are Six 'usual suspects'

These four tend to sit on specific frequencies for long periods of time.

Cyprus (… (more)

Multiple "Sweepers" on my Cascade

G8JNJ

December 2019

It's Over The Horizon Radar (OTHR)

https://en.wikipedia.org/wiki/Over-the-horizon_radar

There are Six 'usual suspects'

These four tend to sit on specific frequencies for long periods of time.

Cyprus (… (more)

Multiple "Sweepers" on my Cascade

G8JNJ

December 2019

It's Over The Horizon Radar (OTHR)

https://en.wikipedia.org/wiki/Over-the-horizon_radar

There are Six 'usual suspects'

These four tend to sit on specific frequencies for long periods of time.

Cyprus (… (more)

FAX - CHA Canada 4414KHz

G8JNJ

December 2019

New FAX frequency for me last night that doesn't seem to be listed.

UPDATE

It's Sydney (VCO) Canada and it's in the FAX Extension list I just missed it.

Fax schedule:
1121 UTC 4416 kHz Ice analysis -… (more)

Howdy, Stranger!

G8JNJ

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Questions About the Mini-Whip Antenna

Multiple "Sweepers" on my Cascade

Multiple "Sweepers" on my Cascade

Multiple "Sweepers" on my Cascade

FAX - CHA Canada 4414KHz