PA8W Amateur Radio

Wil, PA8W,  E-mail: PA8W@upcmail.nl           
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Beware of feedsystem loss!


Losses in feedlines, in baluns, in ATU's, a highly underestimated problem!

I stated earlyer that generally I am not looking for 50 ohm resonant antennas because it is a relatively unimportant parameter.
On the contrary; a multiband antenna with a nice clean 50 ohm impedance on all bands would make me very, very suspicious.
A lot of multiband antennas look very nice testing them with an analyser at their feedpoint. 
But in many cases this is due to heavy losses in baluns, transformers, traps, etc.
I don't know of any antenna principle being able of covering two octaves with a more or less constant feedpoint impedance, except maybe a Log-Per or some space-consuming multi-wire (cage- or fan-) dipole, so something must have been arranged in its feeder system.

An example of a lossy antenna that proofs two points at the same time:

1, It shows that 50 ohm match is absolutely no indicator for good performance,
2, It shows that a nice 50 ohm match on all bands may indicate serious feedsystem loss. 

At the start of my Ham-hobby I built an off centre fed dipole in two different versions using a 1:4 ferrite core transformer from a commercial 40m-10m antenna.
This antenna -inspired on the Fritzel FD3- seemed to be a very comfortable multi band design and I read a lot of very good reviews from other Hams.
It has an off-centre feedpoint that should coincide with impedance points of around 200 to 300 ohm on 40/20/10m bands, according to numerously repeated publications on the internet. The feedpoint should be around 33% off one end, or in case of the Carolina Windom version 28% off one end.
Apparently a great idea, but in real life  I noticed that allthough this antenna worked fine for me on 40m, (apart from a poor match) it is a poor performer on 20m, and modest at best on 10m. (even for a beginner like me...)
Well, that all made sense after I modelled the thing:

The 40m current distribution seems to be ok, as well as the pattern on this band.
However, the impedance is much lower than the 200 to 300 ohms indicated by all literature I found on this subject.
I remember I have been struggeling with the transformer ratio to get things right...

The 20m impedance of around 147 ohms is already a lot more in accordance with the internet, but what the heck happened to the radiation pattern?
It has broadside notches of 20dB! No wonder that I was not impressed by 20m performance!

Now look at the 10m pattern, forget about the feedpoint impedance; Look at those deep notches between the lobes!
I didn't ask for a flower shaped pattern! Is this a flower-power antenna?


And how the heck could this antenna present a more or less 50 ohm load after a 1:4 balun and a lengt of coax, according to so many Hams?
Beats me, but after measurements with this same ferrite core transformer in a 1:1 transformer setting one thing got obvious: 
This core has at least 8db attenuation at 28.4MHz, and around 3dB at 14.2MHz.
So it has masked the poor match by heavey losses, and maybe 12 watts out of my 100 watts actually made it to the antenna...
If I would have had a long lossy coax run instead of the few metres in my case, that would have improved SWR even more.

So you see that a lossy balun or matching transformer will probably make the antenna look better than it is, because the SWR seems to be fairly low.
Well, so is performance....

More on toroid cores

I measured a few more toroid cores in a 1:1 transformer setting, with a power meter and dummy load behind it.
I tried different wire insulation, different winding counts, different winding arrangements, but it turned out to be very difficult to build a 1:1 transformer with low loss from 80m up to 10m.
So, from that moment on I am very reluctant to use any high power toroid transformer in my antenna feedsystem, especially if it has to be mounted out of reach, like at the antenna feedpoint. High in the sky, it may eat up 80% of your power and nobody knows...

Building a low loss transformer for 100w or more is a specialist job, way more difficult than putting some bifilar windings on the first toroid you find at a HAM fest.

Bench-test the core before putting confidence in it.

You just hook up the transmitter, followed by the transformer, then a power meter, and at last the dummy load.
You should read almost the same output power with transformer as without.
If it performs poor on the lowest band, a few more windings might do the trick.
If it performs poor on the highest band, the core is probably not suitable for this high frequency.
And when all seems to be ok, apply full power for several minutes to see if any excessive heat is generated in the core.
Generally, if it performs well at high power -this means low loss- there will be no significant heating up.

In all cases, do overdesign your transformer core; any reactance will stress the core much much more than under ideal match conditions.
So don't take a core that will just do the job. Take at least one size bigger.

A nice approach is to take two identical cores side by side and use them as a pignose core; you will need considerably less windings for the same result.
And there's less wire outside the core: The core centre thats where the flux is! 
All wire necessary for passing the outside is in fact non active.
That's why the pignose is a very efficiŽnt core.

Also for a choke balun, the double core pignose solution is quite suitable; the coax is being bent a little less sharp, and there's less coax length needed for the same result. 



Note that I am absolutely no expert in this matter, but I know for sure that in a lot of applications HAMs waste a lot of power in balun, ununs, and transformers using the wrong core, or just the wrong construction.
If you want to read more about it, you may check http://vk5ajl.com/projects/baluns.php
Or read the in-depth information of W8JI  http://www.w8ji.com/balun_test.htm 

Better ways to feed your antenna.

First stop thinking in 50 ohm, unless you use a good ATU directly at the antenna feedpoint.
Even a thinny 300 ohm ribbon is a much better (less losses) feeder than coax, especially when there's a high SWR in your antenna system.
With a good ATU in the shack and a good twin lead you can feed just about anything you put up in the sky, with very low losses.
The best system is the open ladder line, which is quite easy to homebrew.

The feedline in the picture is in fact an extention of the antenna wire, 2,5mm2 so plenty of copper inside.
The 50mm spacers are plastic tubes of a heavy type of summer curtains, which are used in the back entrance opening of the house to keep flying bugs out.
The components of these curtains can be ordered separately, in lots of colours, so I ordered 50 tubes in black.
Costs: 2,75 euro...

I use a black cable tie to fix the spacer:
First I stick the cable tie through the tube, around one wire, back into the tube, and on the opposite side I close it around the second wire.
Pull tight, cut off the excess of the tie and you're done!

For short cable runs or for portable applications I use a heavy type of 300 ohm line, not the flimsy TV kind, but a very tough version made for Ham purposes. This ribbon line is much easier to handle in field conditions, it will not get tangled, and will not get trapped behind other objects.

Just keep the symmetric feeder line at some distance (at least 5x its spacing) from big metal objects, and run it close to your rig into a old fashioned coreless ATU:

The link coupled transmatch:

On the right there's the link coupled transmatch of the brand "Quick and Dirty". I used 5cm diameter PVC drain pipe for the secundary coil L2 (about 50 windings) and 7cm PVC pipe for the primary coil L1. (about 12 windings). All wire is 1,5mm2 solid copper core house wiring.

Small strips of EPP foam are used as spacers to get the two coils separated nicely. The secundary coil has multiple taps of 5 windings apart, starting from both ends. Between the inner taps there's about 10 windings, on these inner taps the antenna is connected in most of my situations.

The primary coil has taps at 5, 7 and 9 windings from the side that is connected to C1. Mostly I use the centre tap.

Capacitor C1 is 2x 450pF, which may be put parallel for 80m.
C2 is 2x 150pF (in my case made of parallel sections of a 5-section capacitor.) Use good capacitors with wide spacing if you want to apply 100w or more without sparking! This is especially the case for C2.

I use the yellow alligator clips for the antenna feedline, which I extended with 15cm pieces of very flexible leads.
Also, you can see two green ones, these are used to shorten the outside parts of the secundary coil, for the higher bands. This is not shown on the diagram. Also, you may leave the center of C2 floating. Use good insulating knobs on your capacitors!

The primary coil is tapped using the black alligator clip, which is connected to the coax inner conductor.

Ok, first, REDUCE POWER when you want to tune.
Use a SWR meter between tuner and transceiver.
Then, find the right combination of taps and capacitor settings to get a nice 1:1 SWR. 

Once you found the right combination, write down your settings for ease of use. You may as well switch these settings with a solid rotating stepper switch instead of the alligator clips.

This Transmatch is a very old design, but still one of the best ways to match a wide range of antenna loads into a 50 ohm rig.

 




Prototype of my transmatch.




Schematic diagram.
Only,  I did not ground the stator section of C2.
Additionally, I connected C1 and C1A in parallel permanently.

Checking for losses here in the ATU is rather simple, since there's only one way energy could leave the system: heat!
Heat generated by ohmic loss in coils and wires, and heat generated by dielectric loss in the insulating materials. 

I feed a constant carrier of 100 watts into the tuned antenna system and after a few minutes I switch off.
After that, I immediately check for any heating of the components. In my case, I only notice just a few degrees of temperature rise of the outer (primary) coil.
This can hardly be more than a few watts. So I can safely assume that the ATU has a better than 90% efficiŽncy, compare that with the 12% efficiŽncy of the ferrite transformer on 10m...

I feed all my fixed antennas with similar systems, also the unbalanced(!) vertical.
There's one smaller version as well for the 20m loop which is used only on 40m and up.

Of course there are better, and surely more fancy ATU's available, but they can't make a very big difference; even if they are virtually 100% efficiŽnt, the tiny improvement can only be measured, and will not yield a better signal report for you.

A fancy ATU may however be much more comfortable to adjust, but that is not a high priority to me.

So this choice I've made using low loss parallel feedlines and low loss ATU's has given me the freedom to almost ignore feedpoint impedance and focus on the best antenna pattern possible. If I can improve performance by shifting a feedpoint, or by adding a second element, I can do that freely, it will have virtually no adverse effect in feeder efficiŽncy.

You will never hear me say "my antenna presents a fine match on all HF bands."

Because a dummyload does exactly that.

73, Wil.


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