Staggered Beverage
A 300m beverage has a nice small front lobe but its F/B ratio is not really good, an EWE outperforms it easily but unfortunately has a broad front lobe. Desirable is a combination of both.
Some good approaches to improve beverage F/B are here and here but their combiner rely on 90° phase shift, not wise for broadband (500-1600 kHz) that would require a stagger distance of 75m at 1MHz.. Dallas Lankford guided me onto the right track and suggested his QFDA combiner, that requires only a distance of 30m.
Initially I used 30m stagger distance and 10m side by side separation. That improved F/B but did not give the expected attenuation of signals arriving from the sides. Thorough contemplating its working principle I concluded that for maximum reduction both distances should be identical, because the phase shifter shifts all signals by the same amount, not only the signals from the rear. That finally gave the wanted attenuation from the rear and from the sides.

Much later after some helpful tips by Neil Kaz I manage to simulate this structure with 4nec2
and found the simulation results pretty close to what I see in reality.

Results:
The left back sidelobe is reduced by an additional 15-20 dB compared to a single beverage. So the total F/B is even larger. This sidelobe was the most annoying for me, as it points to the south-southwest and signals arrive from there during my local morning. The right sidelobe points to east and signals from there are already well attenuated in the morning. To reduced that sidelobe would be nice but requires even more space

It works all over the MW band and this broadband feature is great when you want to record the entire MW band with Perseus.
However some specific transmitters sites from the side are not attenuated, I found that their level differs from antenna to antenna. The reason is unknown.


Staggered beverage 300m

Single beverage 300m





3D plots (oriented at 320° as it is set up here)  





 
Structure of the staggered beverages oriented at 320° angle as installed here  

 


Horizontal field plot overlaid onto the map to demonstrate what is achieved  








LW properties

An interesting feature can be observed on LW. Changing the phase slightly worsens the F/B but moves the null around.
This 3D-plot is for 207kHz and the phase is set so that DLF from the south east is dramatically reduced that Iceland from the north-west becomes audible even during local noon.
I tried it on all other LW channels and all could be nulled




Switching combiner inputs

When exchanging both antenna inputs of the combiner the main direction should be switched by 180°. This can be observed for example on 810 kHz. The strong BBC Scotland is reduced and Makedonija becomes audible at night.

However the 3D-plot shows a a far less ideal pattern. Nevereless it only requires a relay and why not use this feature during live seesions. Altering the phase will also move around the null a bit





Important hints
During the practical setup I learned a number of things that I had not considered initially:
·Both antennas need to be absolute identical and have identical termination resistors and ground rod resistance.  
·They have to deliver the exact same signal, the only one difference should be the phase shift  
·Both antennas have to have 50 ohms impedance to prevent any impedance transformation caused by the 50 ohm coax  

Initially I used a fixed phase shifter and found that some stations were not well suppressed. Also suppression varied with propagation conditions, especially during skywaves. Soon I wanted to have a variable phase shifter, that can be achieved by making C variable from 220pF to 7000pF in 220pF steps. Also I often discovered signal level differences between both antennas. So I added a balance control pot to it. Since the combiner sits near the feedpoint it is remote controlable via StationList. It uses a digital pot with 256 positions allowing very fine tuning that you never will achieve with a mechanical pot.
This remote control makes it much easier to adjust as I can pre-program certain settings, and even allows remote Perseus users to adjust the combiner via the internet.

Results
This arrangement has worked quite well during this winter. I heard a number of new TA stations that were blocked before. One that stands out is XEW-900, it was blocked by strong RAI Milano. With the staggered antennas I heard XEW quite regularly. The other is 800 kHz, usually swamped by bad splatter of 801-Bavaria. Now 801 is down at similar level as 800 kHz and does not annoy any longer



Staggered structure with combiner block diagram  




Combiner board (top side)

Combiner board (bottom side)



The board is connected to the Ant-Selector




  ©  3S design GmbH   Zeiterfassungs-Systeme   Zugangskontrolle-Systeme   BDE-Systeme   Ferienwohnung südliche Nordsee