VHF aerial for home reception only

[Beadle]

And here is the paragraph that shows that it is perfectly legal to listen on maritime channels.

"Am I breaking the law by owning a scanner?

A. No, but it is illegal to use one to listen to frequencies other than general reception transmissions or those parts of the radio spectrum which your transmitting licence, if you have one, allows you to use. You could be prosecuted for this.

If you think about it it would be illegal to ask your mate to "listen on Ch8 and I'll call you later"

The whole licensing thing is a mess

It started out to raise money and protect broadcasts from interference.
It then got mixed up in the security world.
Then fell foul of advancing technology
A raft of quasi political, quasi technical arguments about CB
The was moved from people who understood it to a bunch of civil servants who didn't.

Which is why the issue is so confused, riddled with contradictions and regulations that might once have been relevant but never got removed because people were trying to legislate for things they didn't understand.

 

[prv]

And here is the paragraph that shows that it is perfectly legal to listen on maritime channels.

"Am I breaking the law by owning a scanner?

A. No, but it is illegal to use one to listen to frequencies other than general reception transmissions or those parts of the radio spectrum which your transmitting licence, if you have one, allows you to use. You could be prosecuted for this.

Which of the two exceptions are you suggesting applies?

Pete

 

[Beadle]

"Antenna design does appear to be an imprecise science, though, with trial and error 'tuning' being necessary for optimum performance. "

Antenna design is a very complex and very precise engineering discipline.

Its the construction and installation where things get a bit fluffy

 

[Playtime]

Antenna design is a very complex and very precise engineering discipline.

Its the construction and installation where things get a bit fluffy

OK - exactly how long should a marine band dipole be then . . . and why?

 

[Beadle]

I wouldn't use a dipole

 

[Beadle]

But since you asked:

A dipole is 2 1/4 waves used as a single aerial

A wavelength is given by 300/freq (MHz)

L = 300 / freq (mhz).

So wavelength = 300/156.8

It is necessary to allow for velocity factor which will vary depending upon the material used. - Say 5%

So Length of full wave = 285/156.8

= 1.818M

So half wave = 0.909M

The dipole is a balanced aerial - that is equal currents flow in both halves of the aerial. It is necessary if a coaxial feeder is to be used to fit a BALUN (balanced - unbalanced network) between the two halves of the antenna and the coaxial feeder.

The dipole works by "reflection against ground so to work to best effect it should be installed a whole number of half waves above earth.

A dipole as above will give some 7dB gain - horizontal polarisation and a take - off angle of 32 degrees if placed a half wavelength above ground

 

[Playtime]

I wouldn't use a dipole

You don't have to use it - just design it.

 

[Beadle]

I just did!

 

[Playtime]

I just did!

Overlap!

Looks like just under 18" is the answer then.

Thanks

 

[Beadle]

No problem

The bill is in the post

 

[Playtime]

The bill is in the post

But you didn't tell me anything I didn't already know.

 

[Beadle]

If you would like an idea of how complex these things can get have a look at the link below

http://www.antennex.com/preview/cfa/nab99cfa.htm

My brain hurts after 1st 10 lines

 

[Playtime]

If you would like an idea of how complex these things can get have a look at the link below

http://www.antennex.com/preview/cfa/nab99cfa.htm

My brain hurts after 1st 10 lines

Agreed.

I did 10 weeks of Fields and Waves at Uni nearly 50 years ago, and didn't understand a word of it. Good job it was optional in the exams!

 

[Beadle]

I came to the conclusion many years ago that Mr Maxwell was far too clever for his own, or anyone else's good

 

[Touchwood]

At the risk of sounding unpopular:

At the risk of sounding contentious:

1) You are not allowed to listen in for fun - this is illegal and you would have been told on your course.

As it happens I met my course instructor at the weekend - he assures me there is nothing in the regs to prohibit receive only "for fun" or for any other reason. Can you quote the source for your belief that listening to marine transmissions is illegal?

2) Although useful to know how to bodge an aerial I would suggest you don't unless it is an emergency. Especially if you'll be on channel 16 since this could interfere with someone using it in an emergency.

I don't see how receive only could possibly interfere with transmissions. Can you clarify?