VHF aerial

[dmayes]

My friend has two VHF aerials, one is used for a standard car radio and is backup for the VHF. While fitting some 50 ohm connectors to allow this to happen, I noticed that both aerials have approx 0.7 ohm resistance between the inner and the shield. I expected either an open circuit or at least 50 ohms. They both seem to work but why this very low resistance? What do they have connecting the inner conductor to the shield? is it some low resistance inductor with a higher ac reluctance?

 

[davidpbo]

What you are measuring is DC resistance, not impedance (which is I believe freqency dependent (someone politely correct me if I am wrong)) DC resitance on my aerial was of the order of 1 Ohm as there was a coil consisting of a couple of turns of copper wire in the base to which the screen and centre core are connected, I cannot remember how for sure but think screen went to one end, core to the other and the metre long aerial was tapped off the centre.

The impedance at radio frequency will I believe be much higher.

 

[William_H]

Impedance is a peculiar value which is essentially independant of frequency in most cases.
An antenna hopefully presents a (radiation) resistance of 50 ohms but this is nothing like a DC resistance more like an impedance in that it only appears at the resonant frequency. ie a rsistance which can only be measured by volts and amps at 150 odd megahertz.

This antenna then is best matched by a cable exhibiting 50 ohms characteristic impedance. Again this can only be measured using high frequency AC but at any sufficiently high frequency. The transmitter will have a source impedance which is that impedance at which all the power is carried up to the antenna or at least with minimum losses.

An example of characteristic impedance can be seen in a hydraulic hose. where pressure = voltage, where volume of oil = current.
If you have a very high pressure system then likely the volume of oil willl be quite low. You need a hose that is small in internal diameter but very strong walls. This equates to high impedance.
If you have a system with huge oil flow at low pressure you need a large bore hose with thin walls being OK. This equates to low impedance. Like wise the wires to the spark plugs on your petrol car carry very high voltage but low current. You need a wire with good insulation but thin conductor. By comparison the wires to your starter motor are low impedance ie high current low voltage. All forms of transmission of energy have a characteristic impedance when the consider the ratio of torque to speed or pressure to volume or voltage to current.

Now in an antenna cable the losses are quite great so exact impedance is more important. The RF power tends to be lost by the capacitance between the inner and outer a bit like and added to insulation losses (being greater if the voltage is higher while the inductance of the inner wire tends to lose power a bit like and added to resistive losses being a greeater problem at higher currents. So we find that there is a best compromise between current and voltage to carry the power which can be described as a characteristic impedance. It also happens that if the impedance is not correctly matched we get power reflecting back particularly from the antenna which also causes losses.

Sorry for all the waffle but it might just explain to some what characteristic impedance of a cable is. In fact the whole story is no where near so simple.
Back to the subject your friends car radio may work a lot better with an antenna with no DC short (coil) between centre and shield as this coil is only effective at marine VHF com frequency and will be lossy at VHF broadcast and very bad at MF AM broadcast frequencies. ie a 17 inch dipole with ground plane may work a lot better on broadcast but still OK on VHF marine com.

good luck olewill

 

[Bejasus]

I currently have my VHF and car stereo hooked up to the same Shakespeare base loaded antenna via an am/vhf antenna splitter, however my car radio reception is crap.

 

[dmayes]

So if I convert the spare VHF aerial to a one meter dipole by taking out the tuned coil and making sure the outer is connected to the mounting plate, then it should work OK on the car radio and still be some sort of backup for the VHF. The reason we are using it this way is just because we had two VHF aerials of the same type one of which had a short co-ax lead and they match each other being on both sides of his boat.

 

[William_H]

No a one metre antenna is too long for a quarter wavelength dipole. A bit over 17 inches is a better length for VHF marine com. olewill

 

[Bergman]

Er

"Impedance is a peculiar value which is essentially independant of frequency in most cases. "

This statement is completely at variance with the facts - the impedance of an aerial or any other component is directly related to frequency.

The characteristic impedance of a feeder is only independant of frequency when it is fed by a source and connected to a load that both have a resistive impedance equal to the "characteristic impedance"

"An antenna hopefully presents a (radiation) resistance of 50 ohms but this is nothing like a DC resistance more like an impedance in that it only appears at the resonant frequency."

Again at complete variance with the facts. The whole point of matching an aerial to the feeder is that the aerial at resonance has a resistive impedance ie it "looks" like a 50ohm resistor. Nothing to do with radiation resistance which is a different thing altogether.

"Now in an antenna cable the losses are quite great so exact impedance is more important. The RF power tends to be lost by the capacitance between the inner and outer a bit like and added to insulation losses (being greater if the voltage is higher while the inductance of the inner wire tends to lose power a bit like and added to resistive losses being a greeater problem at higher currents. So we find that there is a best compromise between current and voltage to carry the power which can be described as a characteristic impedance. It also happens that if the impedance is not correctly matched we get power reflecting back particularly from the antenna which also causes losses."

Waffle is the precise term - completely meaningless

"17 inch dipole with ground plane may work a lot better on broadcast but still OK on VHF marine com"

A dipole does not require a ground plane

A dipole for marine vhf will have each element 17 in (or thereabouts) long and must be mounted vertically to match vertical polarisation standard.

Dipoles are normally half wave - to give a low resistive impedance at the centre feed point. To try to work a quarter wave dipole would be very difficult, the feed impedance will have a high capacitive reactance requiring complex matching arrangements.

A singlequarter wave element can be used as a vertical antenna. This will have a low impedance feed - typically much lower than 50 ohms.

It will also require a "ground" against which it is tuned. This may be the true ground, the surface of the earth, or it may be 1 or more quarter wave elements, typically arranged radially.

The very low feed impedance of such an aerial requires some sort of matching network to be used, gamma or delta matching for example. A commonly used alternative is to lengthen the radiating element to around 5/8 of a wavelength which gives a higher (but still reactive) feed impedance which is more easily matched. It also reduces the angle of radiation which can give a marginal improvement in range.

 

[dmayes]

Thanks for the clarification. My understanding is based on 40 year old college work. I understood the wavelength to be the speed of propogation of radio waves (in meters per second) divided by the frequency. VHF is nice and easy so it is 300,000,000 divided by 150,000,000 giving a full wave of 2 meters. I also understood that a half wave element is best, i.e. a one meter length of wire. I thought that a half wave dipole would have a one meter element connected to the inner matched to a one meter element which is connected to the outer shield. Am I wrong here? is the half wave length proportional to the combined lengths of the dipole? I know that I can just put a bit of wire on the car radio to get it to work I was just interested to know if I could use the existing VHF aerial fo that job. What would happen to a marine VHF if the output co-ax was led to a one meter piece of wire with no tuned circuit between the inner and outer? would it damage the output transistors by not having the correct impedance? It would be useful to know that in an emergency can you put a bit of wire in the VHF aerial socket and use it to transmit. What if I subsituted the tuned coil for a 50 ohm resistor as it is done in the terminators for an ethernet co-ax network?

 

[Bergman]

Depends what you mean by best and what you are trying to achieve.

A dipole is easiest to match - at resonance it gives a resistive impedance of around 50 ohms.

Problem is with marine VHF the standard is for vertical propogation which means that the dipole needs to be vertical. To avoid interaction the feeder needs to leave the aerial at 90 degs which creates mechanical problems.

A vertical quarter wave is easier to install but more difficult to match - because the feed impedance is much lower. Plus you need to create some sort of ground plane for it to work properly.

The simplest homebrew arrangement is to use the coaxial dipole. Someone describes it well in this thread. In my view that is the best way to improvise an emergency aerial.

Reception is not so much of a problem - as you note a car radio will work on any old bit of wire. This is because there is a very strong signal in general from broadcast stations so the sig/noise ratio is acceptable - if you try the same technique on weak signals you notice very quickly the difference a properly matched aerial makes. If you use the bit of wire on transmit the power you try to feed into the antenna will be reflected back into the transmitter

Most transmitters today will sense this and will reduce power to protect the output transistors, but will not then transmit a signal.

The things you describe for ethernet are I think dummy loads. Most certainly you can make one for the transmitter, but remember 2 things:

1 The transmitter produces several thousand times the energy that an ethernet termination produces.

2 The power dissipated in the dummy load will produce a hot dummy load. It will not produce a signal in a distant receiver.

All in all unless you have a particular interest in antennas its best just to buy a commercial one.

I used a simple "rubber duck" on my last boat - had it 13 years and it never faltered - always worked very well.

Going back to your original post: the DC reading means nothing. Most commercial aerials are 5/8 wave which gives a higher impedance than 1/4 wave but requires a matching network to tune out the capacitive reacance. It is this matching network that gives you the DC reading.

If it works with a broadcast receiver thats fine - if nt keep it as a back-up and sling a bit of wire somewhere for use with broadcast receiver.

 

[john_morris_uk]

[ QUOTE ]
Impedance is a peculiar value which is essentially independant of frequency in most cases.


[/ QUOTE ] As Bergman says, this is rarely true.

If you have a completely non-reactiove load than impedence will be independent of frequency.

Such a load only exists in theory, and 'dummy loads' which exhibit the same impedence across a VERY wide frequency range are hard to make and expensive. I have one that is good to 10Ghz in my hand at the moment and it was not cheap!

A simple way of understanding impedence is to think of it as the 'effective resistance' that radio frequency encounters.