Is RG6 overlooked too easily?

[VK2OMD]

RG6/U is a low cost coaxial transmission line that is too commonly overlooked for transmitter applications.

I have written some notes that examines loss, power handling, connectors, and tips and tricks that may be of interest to OMs with an open mind.

The article is at http://www.vk1od.net/RG6/index.htm .

I am considering another article that looks at the use of RG6 in a fully split feedline scenario where a separate tx and rx feedline is used right up to the T/R relay at the antenna connector. For example, where a masthead LNA is used, ten dollars worth of RG6 for the rx feedline performs nearly as well from a station G/T perspective as a few hundred dollars worth of LDF4-50A.

Owen

[VK3GDM]

This is a very interesting article!

I have a role of RG6 Quad I used recently for a new TV ant install.

I was thinking about using the remainder for my next antenna project.

Regards
David

[vk3six/a]

In the industry we use LDF4 and FSJ4 for good reason.
It costs more but it lasts ten times as long providing it is assembled correctly in the first place so in the end you end up in the same place.
How much LDF4 and connectors do you want , we have heaps of throw outs following recent upgrades to PNR and DTV.
Most of it goes to the tip.
Reasons: Most hams could not be bothered to come and pick it up on site.
Or they whinge about how hard it is to re-terminate LDF4 et al.
They prefer to take short cuts and use inferior cable or whatever they can afford and fair enough.
As for being overlooked I don't think so it has been around ever since satellite dishes were starting to be erected.
But it has limitations.
RG6 with any moisture ingress causes the aluminium foil to corrode.
We use RG6 on sat feeds at national TV sites and we are seeing premature failures after rain.
Cable might be cheap but the sealant is not.
Even self vulcanising tape often fails in time or is pecked away by cockies.
This is exhasibated by the presence of 24 Volts causing the centre conductor to etch away.
The same would possibly happen to your pre-amp feed with voltage on the centre conductor.
My RG6 loop balun on the M2 is destroyed even though the RD10FB connecting lead is still OK.
This involves a premature and unnecessary removal of the antenna.
RG6 on a sat dish is one thing but 100 feet in the air is another matter.
I never queried the use of it until now and I am faced with a dangerous 100 foot climb or a $500 bill to drop the tower.
All due to $20 worth of cable.
I wont make the same mistake next time and will use FSJ Flexiline for the balun with $25 connectors.
In your case consider using a common RG10DFB by Kyushin good to at least 500 MHZ and superior to LDF4.
Connectors are $10.
It lasts for decades.
Its available at any CB UHF shop and is stocked by GME Electrophone.

[VK2OMD]

In the industry we use LDF4 and FSJ4 for good reason.
...

You raise some quite valid points.

Different cable constructions have different strengths and weaknesses, and there is no doubt that there are disadvantages to foil shields and to foamed dielectric, both found in RG6 and a host of other cables.

Most cables are damaged by water reaching the outside of the shield (Heliax is an exception).

At my station, whilst I install Heliax outdoors without further protection, other cables such as RG6, RG58, RG213 are installed inside PE irrigation tube which it seems the cockies don't attack, at least not yet.

The cockies do attack Heliax, but punching a hole in the jacket doesn't matter at all as the water is on the outside of the shield. Even where they bite a small hole in the copper shield, water does not propagate up or down the cable thanks to the closed cell foam and adhesive attachment to inner and outer conductors.

But for all the advantages of Heliax, it isn't without disadvantages.

As always, the designer's task is to choose materials that best suit the particular installation. My article puts some information before designers on RG6 performance in transmitting applications, information that might assist an informed design.

Owen

[vk3six/a]

Sorry Kordia does not do East NSW (only West from Ivanhoe) you need to talk to BTS BAE

[VK5PJ]

Put the Kettle on Steve,
on my way to collect Heliax

Sorry Kordia does not do East NSW (only West from Ivanhoe) you need to talk to BTS BAE

[VK4GHZ]

Some LDF450 up in Brisbane wouldn't go astray either.

[VK2JDS]

yes , Its good provided its sealed up well , it moves a lot better than heliax on a dish mount or array on a rotator.
I am also in the industry. oodles of people have it on their houses , towers, sat dishes etc, its a proven cable!
as for me? I run 450 and 550 heliax and lmr400 for tx/rx on a few setups on 1296, but for higher bands i run rg6 off downconverters.
73's

[VK2XSO]

The problem is that RG6 is considered too easily.
I do not have anything good to say about RG6 other than it is cheap and you get what you pay for.
RG6 sold only be sold in Go-Lo etc.
As you can see I'm a harsh critic of RG6, and the stuff that is sold pretending to be RG6. So you may de-bias my comments.

The plots you have included are theoretical or specification, but cheap nasty coax often does not meet spec.
Do you have any real world examples of tests you have conducted with this and other cables ?

I'm also not one to ask somebody to perform tests without being prepared to carry them out myself. To many internet trolls just love to waste the time of those doing important research.
http://www.satvg.org/smf/index.php?topic=9.0

I did some tests quite a while ago which involved crimp and compression connectors. There was a claim that the compression connectors noticably out performed crimp connectors. But when they were tested, the difference between them was so small that it was neglibable. We tested quite a lot of cables and connectors and had different people assemble them just to get the widest range of results as possible. We selected the best, and the interesting and published them.
This is the original thread.
http://www.austech.info/satellite-tv-ge ... ctors.html

The plots are fancy but it's an expensive VNA generating them. It's easy enough to conduct equivalent tests with just some simple equipment and a couple of frequencies. Two or threes tests randomly at 10MHz, 28MHz, 50MHz, 145MHz, 440MHz, 1250MHz measuring insertion loss, and return loss (VSWR) into a dummy load.
The best part is that most people have this equipment and they have thier own coax they can test to compare.

[VK2JDS]

yes, the stuff that is sold as RG6, good point
very variable there Trash
big difference with the quad shield cable, lots better
hey when are you coming out our way again ?, sofala gear?
73's

[VK2OMD]

The problem is that RG6 is considered too easily.
I do not have anything good to say about RG6 other than it is cheap and you get what you pay for.
RG6 sold only be sold in Go-Lo etc.
As you can see I'm a harsh critic of RG6, and the stuff that is sold pretending to be RG6. So you may de-bias my comments.
...

If you read my original post, it was directed at those with an open mind.

I tried to read and understand your linked article, but none of the links worked, and the article seemed based on some archetypal ham myths about reflected power returning to the transmitter ("The simplest method for testing an antenna is using a transmitter and an SWR meter. Basically it compares how much power is transmitted into the antenna compared with how much power is not transmitted and thus reflected and returned to the transmitter."). One is left with the question what happens to this power that "returns to the transmitter"? In the context say of a 60 ohms half wave HF dipole over real ground, fed with a half wave of 600 ohm feedline where the VSWR is 10, Return Loss is 1.7dB, and a nominal 100W transmitter sees a 60 ohm load and delivers close to 100W into that load, most of which reaches the dipole feedpoint and most of which is radiated, does the fact that the feedline VSWR is 10:1 mean that this is a bad antenna under your described test regime?

As far as the apparent point that cable might not meet spec, I cannot, do not, and would not make representations about how any cable that does not meet specification might perform.

Owen

[VK2XSO]

yes, the stuff that is sold as RG6, good point
very variable there Trash
big difference with the quad shield cable, lots better
hey when are you coming out our way again ?, sofala gear?

Hehe, that was quad shield I was describing. We tested a full range of RG6 cables including different manufacturers.
All of them failed. Quad shield doesn't impress me in any cable. What was wrong with a single shield and doing the job properly first time. Aluminium foil and pathetic excuses for braid, really don't cut it.

But everything has its place. RG6 works just fine for satellite TV installations. There seems to be a trend that users now use this cable for terrestrial TV and have forgotten RG59, mostly based on flase impressions that RG6 is much better.

I should be in the area again this week. I have a terrible memory so you'll have to refresh my memory, but I suspect it has something to do with unistrut. If I'm around bathurst, I'm usually listening to the 439.025 heathcote link.
If it's something else, I've forgotten, email me.

I just checked the links... both are working fine. The first is on the satvg forum http://www.satvg.org and the austech server was down over the weekend, so this might explain why the link was not working.

does the fact that the feedline VSWR is 10:1 mean that this is a bad antenna under your described test regime?

No it doesn't, because the antennas is not in question nor under test. A dummy load is calibrated and then the cable under test placed between the load and the test equipment. If it the theoretical cable has a return loss of 1.75dB, this is EXTREMELY poor cable. If 100W is injected into the cable, then almost 67 watts is returned to the test equipment and 33 watts to the load/antenna (4.81dB insertion loss).

This is the importance of considering transmission lines and antennas seperate. CBer's SWR their antenna and coax. (ha cutting the coax to tune the system). Hams know that the transmission lines and antennas are seperate entities, but choose to ignore this in exchange for convenience.

I thought this thread was about RG6. It is an over rated, poor quality, poor performing cable that is frequently manufactured below the already low RG6 standard. It's only redeeming factor is that it is very cheap.

[VK2OMD]

I just checked the links... both are working fine. The first is on the satvg forum http://www.satvg.org and the austech server was down over the weekend, so this might explain why the link was not working.

Your reference at http://www.satvg.org/smf/index.php?topic=9.0 has several links to other articles, but four of the five links do not work.

The link to "A table of VSWR and insertion loss" is actually to a return loss to VSWR conversion table. Insertion loss is not the same thing as return loss, which leaves a question of whether you mean insertion loss or return loss when you use the term insertion loss.

It is perhaps a lack of understanding that leads you to write "The correct term is "return loss". The higher the return loss, the worse the antenna is behaving."

Interpretation depends on the your meaning of the word "behaving".

Keep in mind that the energy in a wave traveling along a transmission line that is terminated in its characteristic impedance is totally absorbed by the termination (or load), there is no reflected wave and the return loss in dB is +infinity. In applications where the load is intended to be matched to the transmission line, high return loss is goodness... but you state otherwise. High return loss is synonymous with low VSWR if you consult your own reference.

Owen

[VK2XSO]

Keep in mind that the energy in a wave traveling along a transmission line that is terminated in its characteristic impedance is totally absorbed by the termination (or load), there is no reflected wave and the return loss in dB is +infinity. In applications where the load is intended to be matched to the transmission line, high return loss is goodness... but you state otherwise. High return loss is synonymous with low VSWR if you consult your own reference.

Sorry if there is some confusion, I have the odd typo when working under un-usual conditions.

Ok.. so I'll just restart from here because my brain is back in gear.
Return Loss and VSWR are the same thing. Insertion loss is also related.

"High return loss" or "Low return loss" .... there is a bit of confusion here because I hear both terms used to mean the same thing. Of course I've made the mistake here myself of such double speak. I say "low return loss" and mean the same as you say "high return loss". It may not be the correct speak, but I hear both said to mean the same thing all the time from a variety of people.

We talk of return loss in values from 0dB meaning BAD VSWR, and infinite dB meaning GOOD.
Is the loss refered to as -10dB or 10dB loss. Doh ! Hence the confusion. Is 5dB higher or lower ?
Since the scale is absolute and you cannot have a positive return loss (more power than your transmit) higher or lower don't really carry meaning, but better or worse do. Is 5db higher or lower than 10dB, is it negative dB or loss.
We both know what the politically correct speak is.

So just to recap.

If we inject a 0dBm signal into the coax, then all of the power should be transmitted to the load and absorbed there.
A power meter at the other end of the coax will read 0dBm.
A power meter looking at the reflected power will see, well nothing but its own thermal noise lets say -120dBm.

Now if the coax has a 3dB return loss, -3dBm signal will be transmitted to the load (insertion loss) and -3dBm will be reflected to the transmitter (return loss).

[VK2OMD]

...
Is the loss refered to as -10dB or 10dB loss. Doh ! Hence the confusion. Is 5dB higher or lower ?
...

An often created confusion, but it needn't be.

Think of Gain as the dimensionless ratio of PowerOut/PowerIn, and Loss as the Ratio of PowerIn/PowerOut. We can express either in decibels by taking 10 times the log of the power ratio.

So, Gain=1/Loss, Loss=1/Gain, or in decibels, GaindB=-LossdB and LossdB=-GaindB.

Real attenuators have less power out than power in, so their Loss is always greater than 1, and their LossdB is always positive.

At HF and above, except for a quite special case, rho (the magnitude of the complex reflection coefficient Gamma) is >= 0 and <=1 and in that case the Return Loss Loss is always greater than 1, and the ReturnLossdB is always positive. (I leave you to ponder the case where rho>1, which implies "Reflected Power" > "Forward Power".)

ReturnLossdB=-20*log(rho) where rho=|Vr/Vf|. (Defined without mentioning phony concepts like Reflected Power and Forward Power.)

A similar quantity to ReturnLoss exists in the S parameter world, it is S11. ReturnLossdB=-S11dB.

It is not unusual to see ReturnLossdB given as a negative quantity. It is usually wrong at HF and above, and is usually a result of the author not understanding the instrument they are using and or the fundamentals outlined above. They read or plot S11dB from a VNA and write it up as ReturnLossdB.

Owen

PS: An example of misleading information from people who should know better is this definition from M2 Antenna's web site:
VSWR - Voltage Standing Wave Ratio is the logrithmic [sic] description of the power the antenna accepts versus the amount it reflects back down the feedline. Synonomous with Return Loss.