Stacking Antennas part two

Due to a big thunder storm, the second part of Stacking Antennas article became a reality. In fact due to this storm a twin of my TH5, badly fell on Parma's roofs, became available. Being in a very bad conditions, it took quite a bit of work to put it back on the air, but it worth the job.

To get to the point, at the end of the previous article we sad we were going to look at the topic in a more practical manner, as making the match transformer, the switching box and so forth.

The usual way to match two antennas in phase place within the same axis is by means of a transformer made of 75 ohms coaxial cable. If the stack must be selectable, the quarter wave sections need to be removed.

In case the antennas individual switching is not a must and only a single band is required, this system is very handy and quick to make. If vice versa the idea is to match two tribanders, it is not suitable since can only be used on the band related to the quarter wave of the 75 ohms coaxial cable, even more if the option is to switch the antennas one by one or both.

The so called UNUN is a broad band unbalance transformer. The main characteristic are: small dimensions which allow switching relais to be placed within the same moderate dimensions matching box. This method does not require a precise coax cable cut of given dimensions.
The only requirement is the exact length of the coax cables going to the antennas, preferably different from a quarter wave length and multiple odds to avoid unwanted matches (multiband antennas are not exactly 50 ohms on each band).
Being the UNUN match ratio constant across a wide frequency range, it becomes possible to easily match ether monobander , log periodic, cubic, tribander, etc.
You do not need to be a Big gun to take advantages from a beams HF Stack, this is probably one of the most efficient upgrade you can add to your contest station, in stead of just buying a linear amplifier, which offers very little benefit on RX.
If the antennas rotates independently, you can also receive and transmit in two different directions simultaneously.

The idea for my BOX came from the "STACK MATCHER" of WX0B, which allows to feed two or three antennas with a single transformer.
In stead of making one "UNUN" with 3:1 ratio to feed all three antennas or one with 2:1 ratio for two antennas, Jay came out with a ratio of 2.25:1 which is a compromise absolutely acceptable.
A "UNUN" transformer 2.25:1 ratio is very simple to build, does not require intermediate taps and can be build to handle enough power.
When is used to match two or three antennas sufficiently spaced, the SWR dip keeps almost the same frequency as per the single antenna alone (it should actually enlarge, NDR).
Personally, on 10M the 1:1 ratio was slightly lower from the single antennas alone.
 

All you have to do is to place the UNUN in a water proof box , better if put half a way from the two antennas (less cable), and feed these last with identical 50 ohms coaxial cable lengths.

By putting the selection relais within the same UNUN box, by means of a remote cable, it is possible switch from the shack among the three possible combinations.
A simple two or three position rotary switch, wired properly, is probably the most practical solution.
With all the antennas fed, relais are not energized (OFF), transformer on the line (through), as you select every single antenna, the relay which by-pass the transformer comes ON (transformer off line) as well as the specific one for the antenna selected.

To save a wire, when a specific relay (upper or lower antenna selected) is ON, it is possible by means of a diode to turn ON also the common one.
 

The way to build the UNUN transformer, consist in winding on a toroid with permeability of 125 or less (for a near 100% efficiency at HF frequency) 5 wraps of trifilar enamelled copper wire.
The wire diameter depends on the handled power (in my case 1,5 mm), fold the tips in order to make connections as shown in the picture

Always check the unit on 25 or 16,7 ohms dummy load, made by putting in parallel two or three 50 ohms dummy loads connected to the output of the unit.
For my test I used the MFJ antenna analizer (thanks to I4WMG) as a generator, and two or three anti-inductive 47 ohms 1/4 Watt resistors in parallel.
With dummy load properly dimensioned, you may use your RTX at the minimum usable power to read the SWR.
The stack works very well indeed, and is easy to find it out , simply switch between the possible combinations, it also allows you to understand the various propagation modes.
Furthermore, the possibility to transmit and receive simultaneously from two different directions is an extra card to play and not to underestimate in order to fully maximize all the operative capability.

See you in the contest, I4LEC