Quarter wave stub bandpass plot

[VK3LU]

Hi all,
Has anyone ever plotted / swept the frequency response of a quarter wave stub of RG6 at 144 mHz.
I would be interested to see how it looks with regard to its proximity to TV RF channel 6 where ch7 is now.

Thanks in anticipation
Nev

[VK2ZIW]

Sorry, but RG6 is THE WORST coax to use, except for satellite TV.
The alfoil does not connect to the braid. Inside an "F" connector there is no
solid connection to the alfoil inner shield. Period.
The anodizing oxide coat on the alfoil is way too good an insulator.
Yes, the braid gets crimped but without the alfoil connected, all hell breaks loose
when you start to use it at 2m.
I have messed around pushing ribbon cable header square pins down between
the alfoil and the plug outer, yes this works, but is it worth it?

At 800MHz plus, the capacitance between the 10mm of alfoil inside the plug
is enough and the DC goes via the braid, so it works for satellite TV.
I would not recommend it for our ~200MHz TV or DAB+ either. (Sydney)

===== Use REAL coax ===== Copper for both inner and shield =====

Alan VK2ZIW

[VK3LU]

Alan,
Where do I get some "real" 75ohm coax that is superior to RG6 in all respects? One that will make a good quarter wave stub at 144mHz.
Incidentally, I did some resistance measurements on some RG6 and got a dead S/C between foil and braid, and yes I kept the foil and braid apart.

Nev

[VK2XSO]

I have done such sweeps of coax.
I'm glad to hear somebody else has finally realised how bad RG6 is.
It's only acceptable use is as satellite TV IF cable, and even that is questionable

Alan, I personally like RG6Q (Quad shield) because if the job isn't worth doing properly once, it's worth doing it four times poorly

Nev, you don't have to use 75 ohm. You can use, well anything. 75 ohm, 50 ohm, 300 ohm ribbon cable, lengths of waveguide, 120 ohm twisted pair, pieces of waterpipe, A couple of star pickets hammered into the ground

OK.. I'm taking the joke a bit far, but if you're specifically looking for 75 ohm coax (for any reason) then RG59 is cheapest choice. RG59... not the stuff that some local retailers sell as "Type RG59".
If the braid falls off when you take the PVC jacket off, it's going to be a poor faux coax.

RG11 is the next step up. It's 75 ohm coax much like RG213.
LDF1-75 or FSJ4-75 are heliax examples that can be difficult to find, but if you know where to look, somebody might give it to you.

I can't seem to find the plots I have of various coax stubs. But they are quite simple and they all follow the same pattern.
There are two types. Short and open. I've made a quick jpg file for you showing the two types.

So the easy way to remember them is that they change ever 1/4 wavelength. pass, stop, pass, stop, pass, stop etc.
The shorted 1/4 wave stubs are STOP (short circut) at DC. Even numbered wavelengths are notched 0/4, 2/4 (1/2), 4/4 (1), 6/4, 8/4 ..... are stop
The open 1/4 wave stubs are PASS (open circuit) at DC. Odd numbers wavelengths are notched 1/4, 3/4, 5/4, 7/4 ....

Now there are some interesting combinations. T shape is a pretty simple quarter wave stub filter. Just to keep it simple we can say the arms of the T and the tail are all 1/4 wave length.
If we add more stubs, the filter notches will become sharper. Stubs can be added to the same point on the T or made as Pi filters. However, if the stubs are not all tuned precisely, the notches will have a lower Q and become more of a band stop or band pass depending on how you set them up.

[VK4CRO]

Just a thought on rg6 - my rx cable from my 2m Kuhne preamp is rg6 , a
28m length as tower is 22m high . this has been in service for more than 10 yrs with no problems.
I used rg6 to achieve some attenuation from the Kuhne , I have good results on 2m dx and dont use the
preamp in my k3 to keep noise levels down .
MAYBE I should try a run of better coax to see the difference
Your thoughts please
regards Cro

[VK3LU]

Gents,
Thanks for your input.
The reason I said I wanted to use RG6 was that it is a simple matter using F connectors and a T adaptor to insert the stub into an existing setup.
If I use, say, RG213 then I have to cobble up something using adaptors. The only way to get around that is to use a small metal box and fit two F sockets and an SO239.
Then again I could use RG59 in an F connector but I would imagine RG6 would have a better notch due to its higher Q than RG59???
I don't want the skirt of the notch encroaching on rf channel 6.
That's why I asked if anyone has plotted a quarter wave stub on 144.

73 Nev

[VK3ALB]

Hi Nev,

If you're trying to keep 2m out of a TV system, I've been there. I've tried a stub made from RG6 on my TV system to trap out 2m. I can't remember the shape but I know it was trimmed with a tracking gen/spec an combo. It was pretty good and had no impact on TV reception. However, it was a bit of a pain to weatherproof and assemble on the TV antenna near the masthead amp - I'm sure you can imagine. In the end I found a much easier solution was to install a Kingray FL3BPMH filter in front of the masthead amp and all the problems went away.

This filter has been discussed previously - I think I can blame VK5PJ for putting me on to it. Price hasn't changed in all that time.

[VK3LU]

Hi Lou,
Weatherproofing is not a problem, it would be mounted in the roof cavity.
My main problem is that I don't have the gear to set it up properly, it would be a case of cut and try.
The Kingray looks an excellent option. I must away and start Googling!

73 Nev

[VK4BG]

Hi Guys,

Last time I looked, Times Microwave LMR-400-75 was available from Rojone..as well as the correct connectors ( but keep it away from your 50 ohm cables...causes all sorts of interesting problems till you realise you have mixed the two...don't ask how I know ).

[VK3ZAZ]

Sorry, but RG6 is THE WORST coax to use, except for satellite TV.
The alfoil does not connect to the braid. Inside an "F" connector there is no
solid connection to the alfoil inner shield. Period.
The anodizing oxide coat on the alfoil is way too good an insulator.
Yes, the braid gets crimped but without the alfoil connected, all hell breaks loose
when you start to use it at 2m.
I have messed around pushing ribbon cable header square pins down between
the alfoil and the plug outer, yes this works, but is it worth it?

At 800MHz plus, the capacitance between the 10mm of alfoil inside the plug
is enough and the DC goes via the braid, so it works for satellite TV.
I would not recommend it for our ~200MHz TV or DAB+ either. (Sydney)

===== Use REAL coax ===== Copper for both inner and shield =====

Alan VK2ZIW

FYI
Better go out an tell all the TV antenna installers as they use RG6 QUAD and there are rolls of it at AWM and Middies.
They also use CRIMP F types.

It is as pointed out used for many sat IF feeds from LNBs to the set top box.
But as for using it for transmitting, can't imagine why unless price is a barrier.
Except M2 who also used RG6 QUAD as the 1/4 Loops in their 6M yagis.
rated at 1000 watts.
Series one no longer used.
I had at least three sets fail.

As for the topic suck out traps/open circuit stubs for TVI suppression they work fine, even can be used to suck out 6M from your HF transceiver when running high power two drops it into the noise, so one tee a 1/4 section and another tee both with stubs qed.

Just get one tx on 6M watch the power from 6M on the tcvr side of the cable to you hf rig
and snip 1mm at a time with side-cutters and you will see watts become milli-watts.

[VK6ZFG]

Hi

Sweep of a 415 mm length (including BNC) of RG6 plugged into a BNC Tee inserted into a short 50 ohm length of coax between 50 ohm ports on a Rigol tracking generator.

146 MHz RG6 Stub

On a 75 ohm feeder system the attenuation may work out to be a bit higher.

Hope it answers the original question.

[VK3LU]

Igor, that's great. Just what I was after.
Good to see that ch6 is not affected by the edge of the notch.
I'll print it out for future reference.

Thanks
73 Nev

[VK2XSO]

FYI
Better go out an tell all the TV antenna installers as they use RG6 QUAD and there are rolls of it at AWM and Middies.
They also use CRIMP F types.

Why do you suppose this is the case?
Because it's the best coax to use for the job?
Well, we know that's not true because there are many better coaxes to use.

There are three reasons why RG6 is used by all the TV installers.
1> It's cheap
2> The best coax for the job based on price and performance had to compete with the price undercut, they did this by making "type RG59" coax.
In other words, they made crappier coax which was worse than RG6. That in itself is an achievement.
So RG59 type coax is worse, but real RG59 is better. Though looking at the insertion loss for each RG6 would appear to be the better coax. The return loss tells a different story.
3> The Bandwagon fallacy - people believe RG6 is the better coax, and in some cases for this is true. But this gets generalised and before we know it there is polarised consensus and it is a self sustaining delusion. It must be good because everybody else says it is.


I did a job many years ago where RG59 was the coax best suited for the job.
However, the customer upon reviewing the design changed this one tiny component to RG6.
The customer was informed that RG6 was not suitable for the job.
The customer insisted because, the customer is always right. He who has the gold makes the rules.
OK - so we installed RG6 at the customer's request.
The project did not work. It had terrible problems and of course the designers and engineers were blamed.
The customer was told what was wrong, but refused to believe that the "better" coax could be at fault.
It was not until we put our balls on the line and replaced the RG6 with RG59 and the project worked perfectly from that day forward.
As a followup we investigated why the customer insisted on RG6. It came down to one of the managers read on a forum that RG6 was superior to RG59.
This cost the customer, the price of the RG6 and it's installation. The cost of replacing it with RG59 and the added bill for fault finding the problem they caused.
This was a multi million dollar project, you can imagine how much just the delay cost them. And all because somebody turned a "trend" into an engineering fact.


If I'm a TV installer I'm going to use RG6 because;
it is cheap and I'll charge you the same price as I was for RG59.
If I don't, my competitors will and under cut me.
My competitors will tell their customers RG6 is better, because it is better for them, not for their customers.
Customers will believe this because it's easier to digest rather than consider they have been short changed.
This confirmation bias will flow against RG59
Anybody who attempts to use RG59 and buys the non-spec coax and confirms it is worse will provide a positive bias for RG6 based on fake RG59. [A=bad ∴ B≠bad]
As long as the install works fine for a year, any fault that develops will generate future income sooner.

Doing a job properly first time achieves job satisfaction. Doing it poorly several times achieves job security.

----

The bottom line is to use RG6 for tasks it was designed for or is best suited or because of it's price.
It is not a replacement cable for RG59 and blindly doing so thinking that it is a better choice may cause problems.
Nor should this be taken as an excuse not to use RG6. Each coax was designed for a job.
Choose the one that is best suited for the task based on it's merits.