Ladder Line

[kb3nyv]

I hope no one minds if I post a "ham" question in this "monitoring" forum. I was wondering what the difference is between 300 and 450 ohm ladder line. I am new to the General Class and am thinking of feeding my HF dipole with ladder line so I can just bypass a balun. Is this an okay plan? I just don't understand why the dipole is 50 ohm but it can be fed with 300/450 ohm ladder line. Thanks

 

[dbox1]

I hope no one minds if I post a "ham" question in this "monitoring" forum. I was wondering what the difference is between 300 and 450 ohm ladder line. I am new to the General Class and am thinking of feeding my HF dipole with ladder line so I can just bypass a balun. Is this an okay plan? I just don't understand why the dipole is 50 ohm but it can be fed with 300/450 ohm ladder line. Thanks

A folded dipole is nominal 300 ohm antenna. A half wave dipole is nominal 72 ohm. A ground plane 1/4 wave with radials is 50 ohm. 450 ohm has less loss over the long haul (500' of 450 ohm has less loss than 100' of RG8) you will more than likely still have to use an antenna matcher at the rig.

 

[iMONITOR]

My concerns might be unfounded, but I would think transmitting any substantial RF power over unshielded ladder-line would not be good for my health. I've seen countless hams that do it, but they all look a little peculiar! :wink:

 

[zz0468]

A few ladder line basics...

Practically speaking, the difference in feeding a dipole with 300 ohm vs 450 ohm line is probably moot. Either one has a very high VSWR.

A properly balanced ladder line should not radiate, in theory. If it's going to feed a radio with a 50 ohm antenna connector, you'll need a matching transformer and a balun of some sort. You can't just hook that ladder line up to the radio.

Generally, when you see a ladder line fed dipole, it's being used as a multiband antenna, and the feedline is tuned with an antenna tuner. You avoid those pesky baluns, and replace it with another, more expensive, piece of equipment. The benefit? Multiband operation. The disadvantage? Theory or not, that open line is going to radiate some. It's VERY difficult to keep it balanced. Those tuners can be quite expensive, too. There are other pros and cons, read about it. There are books...

The dipole fed with open (or ladder) line has a VERY high SWR. That's ok. The antenna tuner makes sure the radio never sees it. If you're looking at just a single band, forget the balun, and feed the dipole with coax. You'll probably never notice the difference. Don't try multiband operation with a coax fed dipole (other than the 40 meter/15 meter trick). Coax won't operate well at all under high VSWR conditions like the ladder line will.

I use open wire 450 ohm line on my antenna, and run it to an SGC autotuner. Works GREAT on all bands.

I hope that helps some.

 

[N_Jay]

My concerns might be unfounded, but I would think transmitting any substantial RF power over unshielded ladder-line would not be good for my health. I've seen countless hams that do it, but they all look a little peculiar! :wink:

Properly matched transmission line does not radiate.
Improperly matched transmission line will radiate.

It does not matter if it is twin-lead or coaxial in design.

 

[iMONITOR]

While we're on the subject of ladder-line, what about for reception/receivers only? I have an Icom IC-R75, and it has both 50 ohm (PL259), and 300 ohm (two posts) antenna connections. It seems almost without exception everyone uses 50 ohm coax feed line. The claim is that it reduces RF noise picked up from inside the home. It makes sense. How would something like TV grade 300 ohm twin-line feed work for HF reception, assuming it's connected to a dipole antenna?

 

[N_Jay]

While we're on the subject of ladder-line, what about for reception/receivers only? I have an Icom IC-R75, and it has both 50 ohm (PL259), and 300 ohm (two posts) antenna connections. It seems almost without exception everyone uses 50 ohm coax feed line. The claim is that it reduces RF noise picked up from inside the home. It makes sense. How would something like TV grade 300 ohm twin-line feed work for HF reception, assuming it's connected to a dipole antenna?

It works well as long as its matched.

 

[kb3nyv]

Thanks everyone for the replies. An antenna tuner isn't really in the budget so I guess I shouldn't be thinking about ladder line. As of now I'm only looking to transmit on one or two bands so I'll just stick with the coax for now.

zz0468 wrote - "If you're looking at just a single band, forget the balun, and feed the dipole with coax."

Won't the coax act as an extension of the antenna without a balun at the feedpoint? Isn't that a bad idea?

 

[N_Jay]

. . .
Won't the coax act as an extension of the antenna without a balun at the feedpoint? Isn't that a bad idea?

If it is matched, it will not, but if it is not matched it will.

 

[zz0468]

Yep... if your dipole is well matched, I doubt you'd be able tell the difference of having a balun or not having a balun. Just feed it with coax. It'll work fine.

 

[k9rzz]

Remember that a 40 meter dipole will load and work fine on 15 meters. Two for one!

You could also make multi-wire dipoles fed with coax for multi band work.

I like using open wire feeders ("window line" these days). Tuners don't have to be expensive and are easy to build if you can find a cheap source of parts.

John K9RZZ

 

[kb3nyv]

Sorry if I'm dragging this on for too long but....
are you all saying that if I have for example a 20 meter quarter wavelength dipole and feed it with 3 ft. of coax or 54 ft. of coax that the SWR should be the same for either the 3ft. or 54ft.?

 

[N_Jay]

Sorry if I'm dragging this on for too long but....
are you all saying that if I have for example a 20 meter quarter wavelength dipole and feed it with 3 ft. of coax or 54 ft. of coax that the SWR should be the same for either the 3ft. or 54ft.?

No.

Because connecting wires under 1/8 a wavelength are usually considered "point connections" and not Transmission lines", and therefore SWR is not relevant.

However, if it is matched, then it makes little difference.

 

[zz0468]

Sorry if I'm dragging this on for too long but....
are you all saying that if I have for example a 20 meter quarter wavelength dipole and feed it with 3 ft. of coax or 54 ft. of coax that the SWR should be the same for either the 3ft. or 54ft.?

That would only be true if you had a perfect match between the coax and the antenna. In reality, there will likely be some mismatch, so if you made two measurements, one with 3' and one with 54' of coax, you would see two different readings. The one with the shorter piece of coax would be closer to being accurate.

BTW, it's a HALF WAVE dipole, not a quarter wave dipole.

 

[kb3nyv]

That would only be true if you had a perfect match between the coax and the antenna. In reality, there will likely be some mismatch, so if you made two measurements, one with 3' and one with 54' of coax, you would see two different readings. The one with the shorter piece of coax would be closer to being accurate.

BTW, it's a HALF WAVE dipole, not a quarter wave dipole.

Oops...HALF WAVE dipole
Let me know if I have this straight; IDEALLY the dipole is going to be 50 ohms and the coax is going to be 50 ohms. A perfect 1:1 match right? So the longer the coax the more loss there will be and higher SWR? So would that be a case where you would want to use a balun--With a longer length of coax?

 

[N_Jay]

Oops...HALF WAVE dipole
Let me know if I have this straight; IDEALLY the dipole is going to be 50 ohms and the coax is going to be 50 ohms. A perfect 1:1 match right? So the longer the coax the more loss there will be and higher SWR? So would that be a case where you would want to use a balun--With a longer length of coax?

If the dipole is "perfect" it will be close to 73 ohms, not 50.

Lets just say it is off a bit, so it is a perfect 50, then hooked to 50 ohm cable is a perfect match and the SWR is the same (0) everywhere.

Now lets say it is a smidge off, so there is a measurable SWR. The SWR is still the same (or very close) everywhere, BUT it will be indicated differently depending on where you measure it. This instrumentation error is due to the fact that you are only measuring VSWR (Voltage SWR) and over the length of the cable the SWR will have different amounts of voltage and current components.

Now if the cable is relatively long, (In terms of loss at the frequency of interest) then the SWR can actually be BETTER further from the antenna (or other point of mismatch) due to the loss of the cable absorbing some of the power in each direction.

An (extreme) example would be a piece of cable with 10 dB of loss either shorted or open at the far end. This "perfect" mismatch would reflect all power, however the SWR measured at the transmitter end would (could) never sow worse than 1:1.01 due to the fact that the reverse power will be attenuated by 20dB from the incident power.

 

[zz0468]

Oops...HALF WAVE dipole
Let me know if I have this straight; IDEALLY the dipole is going to be 50 ohms and the coax is going to be 50 ohms. A perfect 1:1 match right? So the longer the coax the more loss there will be and higher SWR? So would that be a case where you would want to use a balun--With a longer length of coax?

No, you don't have it straight yet. =)

The impedance of a dipole is more like 70 ohms. So, perfectly tuned, you're going to have an swr of around 1.4:1. Your milage may vary, because external influences will affect the impedance - wire size, elevation above ground, nearby objects etc. But in general, there will be some mismatch in a perfectly resonant dipole fed with 50 ohm cable.

You could use 70 ohm cable, and the coax would be better matched to the dipole, but the transmitter is still going to be 50 ohms - you've just moved the slight mismatch to another point in the system. DON'T WORRY ABOUT IT. =) Use 50 ohm coax.

Line length plays a role... with a long enough line, the impedance of the antenna is duplicated at half wave intervals along the coax. A random length piece of coax will have a random swr measurement. But it WON'T be WORSE than the mismatch at the antenna/coax junction. It'll just be different - and inaccurate.

Longer coax=more loss=lower *apparent* swr. This is because the reflected energy from the mismatch at the antenna is attenuated in the coax, thereby fooling the swr meter into measuring less reflected power. The longer the line, the more inaccurate the swr measurement. You can still see where resonance is, but the actual swr or reflected power reading is meaningless beyond that. It is possible with the right (or wrong?) length of coax, to have a bad antenna, and not see it with an swr meter.

The ultimate purpose of a balun is to reduce radiation from the coax. If it's reasonably matched (1.4:1 is FINE), then it won't be much of a problem. If you end up with too much rf in the shack, a balun *might* help. But it certainly isn't necessary to put up a dipole and get on the air.

 

[zz0468]

Clearly, I need to recheck the thread just before I post. N-Jay and I are channeling the same information, except he has a few minute head start.

 

[obijohn]

Oops...HALF WAVE dipole
Let me know if I have this straight; IDEALLY the dipole is going to be 50 ohms and the coax is going to be 50 ohms. A perfect 1:1 match right? So the longer the coax the more loss there will be and higher SWR? So would that be a case where you would want to use a balun--With a longer length of coax?

In the "real world" Baluns are really a waste of money. They were primarily useful to help keep the coaxial cable from radiating signal, and therefore spoiling the radiation pattern of the dipole.

In the "real world" most of us have our dipoles low enough to the ground that the pattern is distorted by "terra firma". Especially any band above 14 MHz. They end up essentially omnidirectional. So this makes the investment in "Baluns" moot.

And the statement that "the longer the coax, the more loss there is, and the higher the SWR" is only half true. "The longer the coax is, the more loss there is" is true. In reality the longer the coax is, the less your ability to measure the true SWR is. It relates directly to how long the coax is.

Confused yet? Welcome to antenna theory. People have spent a lifetime studying this very subject. Look for books by people like ...Maxwell(I forget his first name. Maybe Walt?),He knows his stuff!

Have fun!

 

[obijohn]

Clearly, I need to recheck the thread just before I post. N-Jay and I are channeling the same information, except he has a few minute head start.

Ya got to get up real early to beat out N_JAY on a post...I recommend eating Wheaties!