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VHF areils


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I was going to stay out of this thread. I try not to come across as a know it all, but it seems that is the way it is. But there is just too much inaccuracy going on in this thread (but I say that in a nice way, so no one please, don't take offence) so I will attempt to explain and I will try to keep it simple. But as I said to KM on Friday, when it comes to Antennae theory, it's so damn complex.

Firstly, seeing as it is the last discussion, the ground plate on a Hull has nothing to do with being a "ground" in itself. It would have to be huge if that were the case. It is there for the sole purpose of coupling to the Water.

 

Great path for lightning... OK. Lets take a common frequency, marine VHF channel 16 for example? At 156.800 mhz the wave length is slightly less than 2 metres. A directly fed 1/2 wave dipole antenna would divide phase and neutral equally giving us about 1 metre per radial if centre fed... a resonant antenna allowing for end effect. So you tell me WHERE IT CONNECTS TO EARTH or water?? A 1/2 wave can be made resonant (present a inductive/ capacitive reactance to the transmitter) of 50 ohms just using a coil and capacitor. NO NEED FOR EARTH! Lookup J pole? Its a direct SHORT in the feeder to create a resonant 50 ohm load! Requires nothing connected to it to produce a 1 to 1 SWR.

 

WHEELS also said... The Water is the ground. For HF, where the transmission power can be anywhere from 100W up, the plate is all about getting that power to the Seawater. HF(High Frequency) is AM (amplitude modulation) which means huge swings in Voltage. If you grab a Backstay when someone transmits, it'll deck ya. That same power going out the antennae is also going out the ground plate and the area of a ground plate serves two things. It allows a good conduction area and also stops the water boiling on the plate which causes bubbles and loss of conducting area. However, via capacitance, it is also acceptable to have an earth plate mounted on the inside of a GRP Hull. It will couple to the water on the other side just fine.

 

Ohhh dear!! My back stay I think is about 12 metres and a decent ATU (antenna tuning unit) can, using cleaver electronics to switch in and out various coils, allow me to resonate it just fine from 28mhz to as low as 3mhz using the push pit as earth that is earthed well to the lifelines (on a wooden boat) as the "counterpoise". So touching my pushpit or lifelines during transmit (and I run 400 plus watts) will not send you into conniptions unless you touch the backstay and thats a different story! (smell of burning flesh!!!). HF can be ANYTHING!! FM, PM, SSB, RTTY even DIGITAL!!. ""That same power going out the antennae is also going out the ground plate"". Nonsense! Come and grab hold of my pushpit when I am running 400w!! NOTHING! So what you are saying is?? a 100w transmitter feeding a well tuned antenna is loosing half its power into an earth??

 

WHEELS ALSO SAID...VHF(very high frequency) is FM (frequency modulation).

 

DAM!!! Analogue TV is from 50mhz to 240mhz and is all AM!!! Thats all VHF. Us amateurs do FM on 10 metres and thats HF!! Modulation has N_O_T_H_I_N_G to do with frequency!! (weather satellites transmit AM! and they are UHF!!!) Most voice HF coms are single sideband suppressed carrier although we (hams) are now doing DIGITAL HF!! The reason why we don't have much FM on HF is because it uses so much bandwidth (3 to 5khz voice.. FM TV and sound carrier 7MHZ!! !!). Hams also do SSB on UHF!

 

WHEELS said...It works so differently to AM, you can not compare the two in it's operation and antennae requirements.

 

WHAT?? The "intelligence" in the signal is the modulation. AM, the AMPLITUDE changes, FM the frequency or phase changes. It has nothing to do with the fundamental frequency of the transmitter! (or receiver) OR ANTENNAE!

 

WHeels: Let me go back to the beginning about propagation of the radio wave. It has been bugging me about that 6dB and 9dB thing. Oh and dBi is also inaccurate. dBi is not used because although in theory you could have an Omni directional pattern, in actual practice, it is impossible.

 

It IS POSSIBLE. Its called a 50 ohm non inductive resistor!!

 

Wheels: Firstly because we are working with a Rod of some form. So there is no transmission at the top or bottom of that rod.

 

Really?? Then feel free to put a finger on the top of a transmitting element at even 20 watts!! So, you are telling me that if I transmit from my "rod" and suspend a receiver tuned to that very frequency directly above it or below it, it won't pick up anything??

 

Wheels: What we end up with is a Transmission wave shape looking more like a Donut

 

CORRECT! For a resonator with no parasitic elements. The more "stacked" resonators the lower the donut or higher the gain (over an isotropic)

 

Wheels: This in itself is slightly directional, to a gain factor of 2.1dB over the theoretical omni transmission wave form.Dbd is the term used, which is dB over Diapole. Dipole is the type of antennae we are using.

 

It actually refers to a ISOTROPIC antenna. An impossible theoretically flawless no loss antenna that radiates the input power in all directions. I've got an early start tomorrow so will leave it there. Sorry Wheels old mate, although you are close to the track no cigar!! Yes a 9db antenna will still transmit to the horizon at a lean. The narrower the polar diagram the more RF is being LOST at a lean to the HORIZONTAL! I won't hypothesis as to how much but we track transmitters with high gain antennas ("fox" hunting) and one way is using a directional gain antenna BACKWARDS looking for a null in the signal to give us direction.

I will say it again. NOTHING (at VHF) on a yacht the heels over is beaten by HEIGHT and LOW gain. A 1/4 wave works well as the "donut" is quite elliptical but unless the mast is on an Americas Cup Cat!! (so almost horizontal!!), go for a simple 1/2w antenna. That's why there is so many up there on the masts!

 

Wheels: Anyone still here????

 

YEP!!! :lol:

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Are we not talking Marine here Wetdream?

TV is FM by the way.

I am tired of this thread. I have said my piece. I have been in the Industry for many years and ones can take or leave what I have said. I ain't arguing it.

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Are we not talking Marine here Wetdream?

TV is FM by the way.

I am tired of this thread. I have said my piece. I have been in the Industry for many years and ones can take or leave what I have said. I ain't arguing it.

 

Weeeell... so be it. I have an AOCP and BOCP plus a few other things the doctor said would go if I keep taking the anti-biotic!!...

I ASSURE you non digital terrestrial television in NZ is AMPLITUDE MODULATION ("AM")!!! I quote"

""Given all of these parameters, the result is a mostly-continuous analog signal which can be modulated onto a radio-frequency carrier and transmitted through an antenna. All analog television systems use vestigial sideband modulation, a form of amplitude modulation in which one sideband is partially removed. This reduces the bandwidth of the transmitted signal, enabling narrower channels to be used""

WEBSITE: http://en.wikipedia.org/wiki/Broadcast_ ... on_systems

 

ZL1FOX.

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Take a re-read of your Wiki link and look at "Audio". Are we not talking Voice and are we not talking Marine? I was trying to keep a complicated discussion as simple as possible, which is darn near impossible as it is. Why on Earth are we bringing totally unrelated TV Video transmission into the discussion. Why are we also bringing totally unrelated Antennae into the discussion. Lets please keep this discussion to Marine VHF....which is FM. Nothing to do with the Frequency range of VHF, it is just FM in the way the signal is transmitted.

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Take a re-read of your Wiki link and look at "Audio". Are we not talking Voice and are we not talking Marine? I was trying to keep a complicated discussion as simple as possible, which is darn near impossible as it is. Why on Earth are we bringing totally unrelated TV Video transmission into the discussion. Why are we also bringing totally unrelated Antennae into the discussion. Lets please keep this discussion to Marine VHF....which is FM. Nothing to do with the Frequency range of VHF, it is just FM in the way the signal is transmitted.

 

Point taken (sorry..out with the jandle!! :oops: ). However (there always is one!) people should be encouraged to expand their knowledge, particularly in devices or services that their life may well depend upon! "Dumbing down" can actually be counter productive and usually drags out the professors who want to slap a txt book about! I encourage those reading all this to study WHY radio works and even go so far as sitting a ham license as it will open up a whole lot of the world to them!

 

You're a NICE fella Wheels!! I hope I haven't offended you!

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This might be of interest to some? A great emergency antenna for marine VHF is a "J-POLE". Simple and easy to make out of old TV 300hm ribbon cable. Someone has done a vid on it I found:

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And from the same dude one with a hint of how to tune your basic VHF stainless whip.

 

 

A interesting thread, some well over my head but it's been a goodie all the same, Thanks all.

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