# Quarter wave cables building

1. Jul 12, 2009

### mustafa_2009

Hii Iam new in this field, Please I want to build 1/4 wave cables and cross diodes together to stop high power at 90 MHz frequency for exaample, I want help how can I build some thing like this and what is the equations to choose the right length depending on the frequency....

2. Jul 12, 2009

### negitron

I don't understand what it is you're trying to do.

3. Jul 12, 2009

### mustafa_2009

1)First I want to know about how can one make frequency dependent quarter cables (coxial)(lamdah\4)?
2) Then what is the relation of these cable length with the input power?
3) then I would be able to protect my reciever from reflection high pulse using these cables.

4. Jul 12, 2009

### Pumblechook

It is still not clear.

Do you mean a 1/4 wave coaxial stub?

5. Jul 12, 2009

### mustafa_2009

Exactly !!

6. Jul 12, 2009

### negitron

Well, to start with, the length is only frequency-dependent; power has nothing to do with it. Also, the length is not the physical length, it's the electrical length--to determine the physical length, you need to know the velocity factor characteristic of the particular coax in question. The equation then becomes (lambda /4 ) x VF.

Secondly, stubs are simply impedance-matching devices; they are used to match the impedance of a particular antenna with that of the transmitter or receiver. They aren't used to attenuate unwanted signals. And I've never heard of them using diodes for anything.

7. Jul 12, 2009

### skeptic2

What are the diodes for?

How much power are you talking about?

8. Jul 12, 2009

### mustafa_2009

Really thanks for help,
but I mean is this cables are restrictly frequency dependence, I mean if I have 2 modulated signals (frequency modulation) Low and high frequency the LF with low power and the HF with high power I can reject the high power with rejecting this frequency part.
the diods is used as shunt with short ended to get infinity resistance at the end i mean if we calculate the out pout impedence to be infinity if we have two input ....

9. Jul 12, 2009

### Pumblechook

No, you can use a stub to reject signals. A 1/4 stub with an open end strapped across a coaxial (or open wire) line will present a (near) short circuit across the line. The lower loss (bigger and thicker) the coax cable used in the stub the better the short circuit will be.

The problem is that they may introduce a mis-match at the pass frequency(ies) and have a fairly wide stop band. You can narrow the stop band and reduce any mismatch at the pass F by capacitively coupling the stub to the line rather than direct connection but the attenaution of the undesired signal wont be as high.

Where it is desired to receive signal(s) close to the unwanted F it may be necessary to have 3 or 4 capacitively coupled stubs spaced along the line.

You can make the stubs a little shorter than required and fine tune them with variable capacitors at the open end (between centre conductor and outer).

With very close spacing between received and unwanted f it may be necessary to use a very high Q (and therefore large) cavity selection filter.

I have worked on broadcasting stations where it is required to pick up signals (for re-broadcast purposes) very close to the station's own output frequencies. Stubs are often made of solid copper tubes for that use.

10. Jul 12, 2009

### mustafa_2009

Iam speaking about 20 watt power as high power...

11. Jul 12, 2009

### Pumblechook

The stub length will be 75/f x velocity factor (near to one for air paced and about 0.66 for polyethelene) . So 50/f for poly. f in Mhz, length in metres. You cut long and trim and find tune with some variable capacitance at the open end.

Diodes serve no purpose.

12. Jul 12, 2009

### mustafa_2009

thanks that is really what i search for but the question is how to select the frequency I mean the equations .. which used to know which frequency is selected and deal as short circuit ??

13. Jul 12, 2009

### Pumblechook

14. Jul 12, 2009

### mustafa_2009

thank you ....

15. Jul 12, 2009

### Pumblechook

But unless you are receiving several MHz from the unwanted frequency I suspect a simple stub will not be suitable.

16. Jul 15, 2009

### vk6kro

You can also alternate quarter wave shorted stubs at the wanted frequency with quarter wave open circuited stubs at the unwanted frequency.
This will tend to favour the wanted frequency and reject the unwanted one.
Shorted stubs just have a wire connecting the inner and outer of the coax at the end away from the antenna cable. Keep the stubs away from each other.

The wavelength at any frequency F (in air) is = 300 / F where the wavelength will be in meters.

A quarter wave will be this wavelength divided by 4.

If it is in coaxial cable, it will be this length multiplied by the velocity factor which is a number like 0.66.

So at 95 MHz the wavelength in air is 3.158 M
One quarter of this is 0.789 M or 78.9 cm
Times the velocity factor 0.66 is 0.521 M or 52.1 cm

So, just cut the cable a bit longer than this and snip off bits until it works best.