Second Best location for an antenna tuner #antenna #tune


KI7MWA
 

If I understand correctly, the BEST place for an antenna tuner is at the antenna feed point.  If the antenna is being fed with 50 ohm coax, the feed-line losses would be minimal so long as the impedance at the feed point was also 50 ohms, wherefore any mis-match created by the antenna would be corrected by the tuner, and any reflected energy would be re-reflected immediately back into the antenna...voila, minimal losses!

Barring that, as in my case, wherein my antenna is being fed with 450 ohm ladder line (which, so I'm told, has minimal transmission losses), the 2nd best place would be at the end of the ladder line, just before it connects to the 1:1 current balun  (after which, the coax that connects to my rig) - NOT, as is more convenient, at the place where the coax connects to my rig.

Am I presenting a reasonable degree of understanding?


Michael.2E0IHW
 

Dunno. I once used the following setup :

inverted vee dipole > homebrew ladder line > 4:1 balun outside > short 50ohm coax > AMU inside > TRX

It worked well.

Michael 2E0IHW

On 14/02/2022 18:41, KI7MWA via groups.io wrote:
If I understand correctly, the BEST place for an antenna tuner is at the antenna feed point.  If the antenna is being fed with 50 ohm coax, the feed-line losses would be minimal so long as the impedance at the feed point was also 50 ohms, wherefore any mis-match created by the antenna would be corrected by the tuner, and any reflected energy would be re-reflected immediately back into the antenna...voila, minimal losses!

Barring that, as in my case, wherein my antenna is being fed with 450 ohm ladder line (which, so I'm told, has minimal transmission losses), the 2nd best place would be at the end of the ladder line, just before it connects to the 1:1 current balun  (after which, the coax that connects to my rig) - NOT, as is more convenient, at the place where the coax connects to my rig.

Am I presenting a reasonable degree of understanding?


Roelof Bakker
 

Hi,

Good question and one I have asked myself.

My antenna is a 2 x 16 meter doublet fed with 11 meter open feeder which enters the house in a
fixed cupboard in the living room. Don't ask why!

The shack is upstairs and RG58 coax is run through the ceiling of the fixed cupboard into a
similar cupboard in the upstairs room and further to the radio. All in all about 8 - 10 meter
long.

Talking about convenient, walking downstairs upstairs at every band change is not.

I tried a rf-sensing remote auto-tuner downstairs, but at times it got stuck and required to
be used on a different band first, before it settled at the band of interest.

I am using a 1:4 balun between the auto-tuner and open feeder.

The next thing I did was to compare on all bands 80 to 10 m the antenna current in the open
feeders for:

1. the auto-tuner downstairs and 30 cm RG58 between the tuner and balun.
2. the auto-tuner upstairs next to the radio and 8 - 10 meter coax between tuner and balun.

(Which took quite a bit of upstairs downstairs.)

Much to my surprise and against common theory, the antenna current did not show a signicant
difference on any of the 9 bands used upstairs or downstairs.

My current auto-tuner is a Z-100 plus, located next to the radio and it works.

73,
Roelof, pa0rdt


Jim Allyn - N7JA
 

I used random length wire antennas for several decades before there was such a thing as an autotuner (or at least, I was unaware of their existence if they did exist).  So, the manual tuner was always right at the rig where I could tune as needed.  The best place for a tuner is wherever it allows you to get on the air.  Easy peasy.  At the other end of the scale, the impedance of an antenna will vary with frequency, therefore the SWR will vary with frequency.  The loss in a transmission line will vary with frequency and will vary somewhat depending upon the SWR present on the line.  Then you have the losses in the balun, which will vary with frequency and with the impedances presented to it at either end.  Of course the losses in the tuner will also depend upon frequency and SWR presented to it.  So, if you really want to know the best place for your tuner, you need to know the impedance of the antenna at several frequencies in each band of interest, the loss in the transmission lines at every frequency and SWR that will be presented to it, and the loss of the balun and the tuner at every frequency and SWR (and power level) presented to it.  You could probably do the measurements on all that in a month or two.  Then you could write a program to calculate which combination of transmission lines, balun location, and tuner location and settings results in the best efficiency.  If that sounds like a lot of fun to you, then that's what you should do.  Otherwise, put the balun and the tuner wherever they are most convenient, and get on the air and have fun.  And maybe now and then, when you feel like it, change something in your system and see if it works any better.  Or don't.

But to answer your question, yes you are presenting a reasonable degree of understanding, as far as it goes.  Now go get on the air.


Stephen Sherer
 

The comments from Jim above, are very well said. Put the tuner where you must to get on the air.

However, if you are looking for how to have the most efficient system, high signal to noise ratios, good radiation patterns from your antenna and no common mode currents in the shack equipment,  use as little coax as possible. Don't use any coax for the transmission line run from the antenna to the shack. Most of your losses affect signal-to-noise ratios and the bulk of your losses will be in the coax for the reasons Jim mentioned above. Use RG8 coax, yes the big stuff, to interconnect your equipment inside the shack and nowhere else. Yes, you can use the smaller diameter coax, with more losses. All the little losses in the "system" add up before you realize it and become unknown and hidden larger losses all of which affect the signal to noise ratios when receiving.

I use RG 8 feedline in the shack for this reason. These losses I mention, affect your ability to hear weaker signals because the losses manifest themselves in lower signal-to-noise ratios. Weaker signals will be covered up by the noise and hidden just from using lossy coax. (Use the highest quality coax you can get). When you use coax, this is where you spend the money. As such, limit your coax use to the interconnections of equipment inside the shack only, and then spend the money on LMR 400 coax for those interconnections, The most efficient coax you can buy. You will never know what you are missing when less efficient coax is used. Use the highest quality coax you can get. 

All of the aforementioned is to manage and maintain the highest signal-to-noise ratios possible. This is what an antenna system is all about when we speak of efficiency in the system. It's about managing losses in the entire system. Look at it in two ways, we want efficient radiation of our own signals and we want high efficiency for receiving signals. The two are interconnected.

I use 600-ohm parallel line from the 80-meter loop antenna feedpoint, where I have a second 1:1 current choke balun, to the feed-through panel at the shack window. Inside the shack window, right next to the window feedthrough panel, I have mounted indoors, the 4:1 current balun. Here is where I use RG8 coax only, from the 4:1 current balun to the 1:1 current choke balun inside the shack and then to the connections with the tuner and from the tuner to the radio or any other accessories I use.

As we are talking about the need for choke baluns for controlling common mode currents and antenna efficiency as it is related to high signal to noise ratios, all of this will be for naught, if the station is not grounded properly. I mention this because there is much conflicting information available on how to ground the station. If this is not done correctly all the money spent on the choke baluns and top-shelf coax will not matter.

Use as little or no coax to the shack feedthrough panel as possible. Sometimes it cannot be helped. but keep the coax lengths under ten feet if possible.

I want to mention something else. You are most likely using your wire antenna for multiple bands and as Jim mentioned with each band there will be different SWR for each band and more or less common mode currents that will change with the band in use. These common-mode currents become a big problem fast when using parallel transmission lines and will be introduced to all of your equipment.

The antenna and the parallel line used to feed the antenna must be decoupled from the shack equipment. Install a 1:1 current balun, also referred to as a RF choke between your last piece of equipment in the chain, inside the shack and the 4:1 current balun. This will choke off the common mode currents.

A 4:1 current balun will not decouple the parallel line and the antenna from your shack equipment nor will it stop the common mode currents.

Only a 1:1 current balun will do this.  The 1:1 current balun, also called a choke balun, needs to be placed between the 4:1 balun and the last piece of equipment, usually the antenna tuner, in the chain leading to the 4:1 balun. The 1:1 current choke balun will stop the common mode currents and send them back to the antenna to radiate or if you are using a long wire or any antenna that uses ground radials or counterpoises of some kind, this will force the common mode currents to flow down the counterpoise wires or ground radials, if any are used, depending on the antenna.

You may have misspoke in your opening comments about the second-best place for placing the antenna tuner, "between, the 1:1 choke balun and the 450-ohm ladder line."  This is not the correct position.

Place the 1:1 current balun between the tuner and the 4:1 current balun. These two should be in series with nothing between them then, connected to the 450-ohm ladder line, otherwise, all those common mode currents will be inside your tuner and then pass to the rest of the shack equipment.

As I believe you are wanting to have the most efficient "system" by managing the highest possible signal-to-noise ratios with good clean audio. One more consideration.

Theoretically, the best setup would be, to place a 1:1 current choke balun at the antenna feed point and then connect the 450-ohm ladder line to the 1:1 current choke balun at the antenna feed point. This keeps the bulk of the common mode currents at the antenna where you want them. Then you will also still need a second 1:1 current choke balun at the shack, between the last piece of equipment, usually the antenna tuner and the 4:1 current balun used for impedance transformation as described above. The reason is that when you are transmitting, the 450-ohm ladder line will have newly induced common mode currents from your antenna while you are transmitting. This is why, when using parallel line of some kind, a 1:1 current balun needs to be used at "both ends". The need for using two choke baluns will not become apparent until you are using more than 100 watts, usually. This does not mean that you should wait until you notice a problem. Even at lower power, use a 1:1 current choke balun at both ends of the transmission line. This decouples the antenna from the transmission line, the station equipment from the transmission line and returns the common mode currents to the antenna and or ground radials or counterpoise wires. Remember just because you don't see symptoms of common-mode currents, does not mean they are not there. They are there and need to be managed.

The "System" I speak of is the combination of the station equipment, the transmission line and the antenna. They must be thought of as one system

That having been said. Without proper station grounding, all the money spent on top-shelf coax and choke baluns will not matter. I bring this up because there is so much conflicting information about station grounding.

To simplify, do not use a ground bus behind your station equipment and then connect your ground braids from each piece of equipment to the ground bus.

Connect the ground braids from each piece of equipment to one central point at the back of the antenna tuner, the antenna tuner ground lug, and this includes an amplifier if you are using one. From the ground lug of the antenna tuner, then route a final ground braid to the feedthrough panel and on, to the ground rod outside the shack.

If grounding the station in this manner creates problems in your shack, this means you have just brought to light the real problems that have been hidden.

KE4LJH
Steve, 
Florida