7dB (colinear) antenna question

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Re: 7dB (colinear) antenna question

Post by Admin on Mon Mar 06, 2017 6:03 pm

Hi Alex,
Absolutely no problem:

As you are aware, light, electromagnetism and radio (all the same thing) travel at 299.7 m/us - IN AIR. If you stuff a signal into a fibre optic or coaxial cable then things change. A coaxial cable has both inductance and capacitance uniformly along it's length.

If you were to connect a 9v battery to a bit of coaxial cable then the inductance will oppose the change from 0v to 9v, and the capacitance will take time to charge up from 0V to 9V. You can think of a coaxial cable as a long string of series-connected inductors that are grounded at every junction by capacitors.

When you connect your battery, then the voltage at a distance will begin to rise, slowly, until the first capacitor after the first inductor, will charge. This is then connected to the next inductor that will also oppose the changing voltage rise, and the next capacitor will also begin to charge. This ripples down the cable, until eventually the voltage at the other end will become 9V, but it will take time.

The effect is that if you measure the end of the coaxial cable then the voltage from your 9V battery will suffer a delay. This delay is called the Velocity Factor, abbreviated to VF.

This is an over-simplication since there is not a collection of inductors, but a long single inductor with one long continuous capacitor.

As the diameter of the cable increases, so the capacitance falls, and it is the ratio of the inside of the outer-conductor, and the outside of the inner-conductor that governs the characteristic impedance of the cable. the propagation delay (VF) will fall.

A 1/4" diameter coaxial cable will have a velocity factor of about 0.66 (66%) and a 1.5" Diameter cable may be as high as 0.85 (85%). Twin-wire feeder has a VF something like 0.95 (95%).

So if you cut a bit of coaxial cable to make a 1/2-wave balun for, say, 100MHz, then 150m/100MHz = 1.5 metres (150 is half 300m/us). But because of the VF, the physical length of 1.5m is too long. Multiply 1.5m x VF (0.66 in our example) = 1metre.

Proof: Get a bit of URM76, RG58, etc and cut it to 1-metre length. Short-circuit both ends with a small 1cm loop. Now measure it using my GDO2 coupled to one of the loops and you will see a massive dip at 100MHz, NOT 150MHz. You can use this effect to measure the VF of a bit of coaxial cable.

If a 1/2-wavelength of coax is shorted at both ends, then it must have a low impedance termination at both ends, and a high-impedance in the middle. 0-volts and max current (low impedance) occurs at 1/2-wave intervals from the feed-point. Maximum voltage and no current (high impedance) occurs at 1/4-wave, 3/4-waves, 1.25-wavelength, 1.75, etc.

Have I explained this simply?

Best regards from Harry - SM0VPO

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Re: 7dB (colinear) antenna question

Post by Jalex2 on Mon Mar 06, 2017 4:59 pm

Hi Harry
I have a quick question here too.
Can you explain velocity factor? 
I haven't done much antenna work but I find the capacitance for a given length
of coax varies greatly depending on what coax you use. I use it
mostly to keep noise out of sensitive circuits and the capacitance is 
sometimes problem.

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Re: 7dB (colinear) antenna question

Post by Admin on Mon Feb 27, 2017 5:24 pm

Hello again Axxxxx,
Sorry about the delay but I am sometimes busy in the evening with loads of "haftas" that have to be done, so I cannot always read e-mail. Tomorrows hafta is a company party, with loads of food and drinks :-)

As regards your question , there is absolutely no problem. I chose RG58 because I have a lot of it, but in principle you can use any coaxial cable, even the miniature cables that are better to coil up and more easily hide. The only thing you need to be concerned about is the cable "velocity factor" (VF).

RG58 VF is about 0.66, which affects the length of the 1/2-wave phasing. 150/frequency is a 1/2-wave, then multiply this by the cable velocity factor and you will have the correct length. This works for any coaxial cable. If you want to be very accurate you can cut the coax a little too long and check the antenna using an antenna analyser, VSWR, then shorten the coaxial cable until you get the lowest VSWR centred at the frequency you want.

Alternatively, you can use a Grid-Dip oscillator to check the cable resonance by shorting one end in a small loop and checking for resonace at HALF the frequency you are interested in. Then you can clip the cable until you get exactly 1/2 the correct resonant frequency (with only one end shorted it acts as a 1/4-wave).

Have I answered your question satisfactorily? If you have any further question then please do NOT hesitate to contact me, preferably via http://sm0vpo.forumstopic.com. I shall post your question to the forum because this is a good question that could help other people who wish to modify the design.

Very best regards from harry Lythall - SM0VPO

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7dB (colinear) antenna question

Post by Admin on Mon Feb 27, 2017 5:22 pm

Dear SM0VPO,

I was going through your "7dB antenna". You can assume I am a Novice in Stacked antenna.
My question is for Phasing lambda/2 , is it necessary to use rg58 kind of  50 ohm cables only? or I can use rg59 ( I have plenty)

Please tell me if i rg59 can be or can't be used with bit description.
I want to learn Smile

de VU3XXX

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