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The Importance (or not) of being resonant #elmer

Jef - N5JEF
 

I've had a couple people from this list contact me directly, asking how I can say that SWR and resonance have nothing to do with the efficiency of an antenna, when they hear almost everybody in the ham world talking about how important it is to trim you antenna to resonance, and get the lowest possible SWR to know that your antenna is radiating efficiently.

I came across this article that explains it quite clearly:

FYI,

- Jef  N5JEF

Alan Thompson
 

Great read - Food for thought.

I put up a closed-loop antenna cut to a full wavelength at 160 meters and off the ground about 60'. I feed it with a piece of 450 ohm ladder lane that runs all the way into an ATU that sits on my desk. I can confirm that I've been able to work 160, 80, 40 and DX on 20 meters into the south Pacific and the Ukraine with 100 watts fed into this setup. Haven't modeled it, and no doubt the radiation patterns are wonky across the bands, but it can and does work.

Best Regards,

Alan Thompson -  W6WN

Jef - N5JEF
 

Alan -

Your full wave loop ought to do a good job for NVIS on 160, being at just about the lower limit for effective height at 1/8 wavelength.
Then at 80 meters it should be good in all directions, but with a quite low takeoff angle, so very good for 80 meter DX but not so good for local/regional.
Then at 40 meters, it'll split into a clover leaf, with higher gain for great DX in some directions, and not much at all in others.

I have an 80 meter full-wave loop here at about 30 ft.  Very happy with it on 80 and 40 for solid coverage of the west coast.
I need to get back to work on my 10 ft diameter loop...

- Jef






On Thu, Aug 8, 2019 at 7:06 PM Alan Thompson <alan@...> wrote:
Great read - Food for thought.

I put up a closed-loop antenna cut to a full wavelength at 160 meters and off the ground about 60'. I feed it with a piece of 450 ohm ladder lane that runs all the way into an ATU that sits on my desk. I can confirm that I've been able to work 160, 80, 40 and DX on 20 meters into the south Pacific and the Ukraine with 100 watts fed into this setup. Haven't modeled it, and no doubt the radiation patterns are wonky across the bands, but it can and does work.

Best Regards,

Alan Thompson -  W6WN

Alan Thompson
 

Thanks again for the insight. That's pretty close to what I've run into. I have it on my list to plot my loop to see what it's really doing but, for now, it just seems to work really well, and it was cheap!

At first, I was worried about RFI in the shack using ladder line. But that link you sent seems to bear it out that the feed line is the biggest variable in system losses, and that ladder line is the preferred, lowest-loss choice over coax. 

Other information I've found seems to tout ladder-line, as long as the length avoids odd multiples of a 1/4 wavelength on desitred frequencies.
.
Best Regards,

Alan -  W6WN

Jef - N5JEF
 

On Thu, Aug 8, 2019 at 8:52 PM Alan Thompson <alan@...> wrote:

At first, I was worried about RFI in the shack using ladder line.

Coax and ladder line will both radiate if used the wrong way.

The main advantage of ladder line is its much lower loss, which can very useful with the range of SWR that goes with broadband antennas. As long as its kept balanced, it doesn't radiate.

The main advantage of coax is its convenience of routing due to its shielded design.  But it's unbalanced, so any common mode current will flow along the _outside_ of the shield and radiate, conversely, be picked up as noise in the receiver. 

But that link you sent seems to bear it out that the feed line is the biggest variable in system losses, and that ladder line is the preferred, lowest-loss choice over coax. 

They both have their place.  Coax is convenient, and works well with narrow band antennas (and it should have a balun, if you're feeding the unbalanced cable from a balanced antenna like a dipole.) 

Ladder line can be less convenient to route, but along with an antenna coupling device, it provides low-loss broad-band or multi-band operation with balanced antennas such as such as your full wave loop, or a doublet such as the double-extended-zepp. 
 

Other information I've found seems to tout ladder-line, as long as the length avoids odd multiples of a 1/4 wavelength on desitred frequencies.


There's a lot of mythology circulating about magic lengths of ladder line or coax.  Either type of transmission line can be used to transform impedance at a _single_ wavelength (plus any multiple of a half wave). This is an inherently narrow band technique, but it has been used in some cases as a compromise solution as with G5RV design, to redistribute impedance mismatches so that for certain bands they fall within the range of a limited < 3:1tuner built in to some radios.  

With a good tuner these multiples have no practical significance.

By the way, my 80 meter full-wave loop has the automatic antenna coupler (SG-230) at the antenna feed point, then a balun, then coax run underground back to the shack.

- Jef

Brian Gohl - AI6US
 

If your wish is to have a dipole antenna fed with ladderline and present a auto tuner with lower impedance levels (for ease of matching) across multiple bands, the feedline length should be cut at an odd multiple of 1/8th wavelength for each frequency range that you wish to work. I found that the feedline length trimming to have a significant impact on the ability for autotuners (internal 3-5:1 or external wider range autotone units) to reliably and quickly tune.

For example: on my 80m doublet, the external MFJ 993b and LDG Z11 Pro2 would not reliably tune several bands when the ladderline length was at the starting 130' length. Once I trimmed the length back to a calculated odd multiple of 1/8th (around 108') all bands reliably autotune.

To support what Jef suggests, using a manual Dentron Super Tuner, The length of the feedline or the length of radiator wires make little difference as the tuner accepts almost any mismatch value.

Here is a great document that I have found helpful in making a multiband doublet and calculating the feedline length (pages 3 and 4): 
www.orarc.net/MultibandCenterfedZepp.pdf
--
Best Regards!
Brian Gohl - AI6US
(916) 770-7751


-------------------------

On Fri, 09 Aug 2019 10:24:02 -0700, Jef - N5JEF <jef@...> wrote:

On Thu, Aug 8, 2019 at 8:52 PM Alan Thompson <alan@...> wrote:

At first, I was worried about RFI in the shack using ladder line.

Coax and ladder line will both radiate if used the wrong way.

The main advantage of ladder line is its much lower loss, which can very useful with the range of SWR that goes with broadband antennas. As long as its kept balanced, it doesn't radiate.

The main advantage of coax is its convenience of routing due to its shielded design.  But it's unbalanced, so any common mode current will flow along the _outside_ of the shield and radiate, conversely, be picked up as noise in the receiver. 

But that link you sent seems to bear it out that the feed line is the biggest variable in system losses, and that ladder line is the preferred, lowest-loss choice over coax. 

They both have their place.  Coax is convenient, and works well with narrow band antennas (and it should have a balun, if you're feeding the unbalanced cable from a balanced antenna like a dipole.) 

Ladder line can be less convenient to route, but along with an antenna coupling device, it provides low-loss broad-band or multi-band operation with balanced antennas such as such as your full wave loop, or a doublet such as the double-extended-zepp. 
 

Other information I've found seems to tout ladder-line, as long as the length avoids odd multiples of a 1/4 wavelength on desitred frequencies.


There's a lot of mythology circulating about magic lengths of ladder line or coax.  Either type of transmission line can be used to transform impedance at a _single_ wavelength (plus any multiple of a half wave). This is an inherently narrow band technique, but it has been used in some cases as a compromise solution as with G5RV design, to redistribute impedance mismatches so that for certain bands they fall within the range of a limited < 3:1tuner built in to some radios.  

With a good tuner these multiples have no practical significance.

By the way, my 80 meter full-wave loop has the automatic antenna coupler (SG-230) at the antenna feed point, then a balun, then coax run underground back to the shack.

- Jef




--
Brian- AI6US

Jef - N5JEF
 

Brian makes good comments and the paper he linked makes good sense.

Here's another paper, by Cebik, with even more detail.  If you digest this, you'll have a very good understanding.


In it, Cebik says "Hence, it is very difficult (although not impossible) to design a feedline system so that on each band we end up with just about the same impedance at the shack entry. Most amateurs let the antenna tuner do the work of transforming whatever impedance appears at the terminals to the transceiver's required 50 Ohms."

I have two of the SGC SG-230 antenna couplers, and they have been able to quickly and efficiently transform any impedance I've thrown at them.  That way, I don't spend any time calculating or cutting or trying to figure out how to route a certain length of transmission line to keep it straight down and perpendicular to the doublet (regardless of height in the field) and not too close to the ground between there and my tuner...

The SG-230s have a great reputation in marine HF, and they can often be found in good used condition on ebay for about $300.

- Jef

On Fri, Aug 9, 2019 at 1:27 PM AI6US <ai6us@...> wrote:
If your wish is to have a dipole antenna fed with ladderline and present a auto tuner with lower impedance levels (for ease of matching) across multiple bands, the feedline length should be cut at an odd multiple of 1/8th wavelength for each frequency range that you wish to work. I found that the feedline length trimming to have a significant impact on the ability for autotuners (internal 3-5:1 or external wider range autotone units) to reliably and quickly tune.

For example: on my 80m doublet, the external MFJ 993b and LDG Z11 Pro2 would not reliably tune several bands when the ladderline length was at the starting 130' length. Once I trimmed the length back to a calculated odd multiple of 1/8th (around 108') all bands reliably autotune.

To support what Jef suggests, using a manual Dentron Super Tuner, The length of the feedline or the length of radiator wires make little difference as the tuner accepts almost any mismatch value.

Here is a great document that I have found helpful in making a multiband doublet and calculating the feedline length (pages 3 and 4): 
--
Best Regards!
Brian Gohl - AI6US
(916) 770-7751


-------------------------
On Fri, 09 Aug 2019 10:24:02 -0700, Jef - N5JEF <jef@...> wrote:

On Thu, Aug 8, 2019 at 8:52 PM Alan Thompson <alan@...> wrote:

At first, I was worried about RFI in the shack using ladder line.

Coax and ladder line will both radiate if used the wrong way.

The main advantage of ladder line is its much lower loss, which can very useful with the range of SWR that goes with broadband antennas. As long as its kept balanced, it doesn't radiate.

The main advantage of coax is its convenience of routing due to its shielded design.  But it's unbalanced, so any common mode current will flow along the _outside_ of the shield and radiate, conversely, be picked up as noise in the receiver. 

But that link you sent seems to bear it out that the feed line is the biggest variable in system losses, and that ladder line is the preferred, lowest-loss choice over coax. 

They both have their place.  Coax is convenient, and works well with narrow band antennas (and it should have a balun, if you're feeding the unbalanced cable from a balanced antenna like a dipole.) 

Ladder line can be less convenient to route, but along with an antenna coupling device, it provides low-loss broad-band or multi-band operation with balanced antennas such as such as your full wave loop, or a doublet such as the double-extended-zepp. 
 

Other information I've found seems to tout ladder-line, as long as the length avoids odd multiples of a 1/4 wavelength on desitred frequencies.


There's a lot of mythology circulating about magic lengths of ladder line or coax.  Either type of transmission line can be used to transform impedance at a _single_ wavelength (plus any multiple of a half wave). This is an inherently narrow band technique, but it has been used in some cases as a compromise solution as with G5RV design, to redistribute impedance mismatches so that for certain bands they fall within the range of a limited < 3:1tuner built in to some radios.  

With a good tuner these multiples have no practical significance.

By the way, my 80 meter full-wave loop has the automatic antenna coupler (SG-230) at the antenna feed point, then a balun, then coax run underground back to the shack.

- Jef