Currently viewing the tag: "Rhombics"

New Shack almost completed

By On · Leave a Comment · In Amateur Radio

One operating position is fully functional.   The shack has 4 operating positions and each position can choose any available transmit antenna through a 100 kW Strip Line Matrix Switch remotely controlled.

The remaining tasks are to install panels at each operating position to plug in bandpass filters, install wiring for remote antenna switches and power combiners, install computers at positions C and D, wire up PTT control to the Strip Line Matrix switch (to prevent the possibility of hot-switching) and build an indicator panel for the switch.

I will now concentrate on the farm work until autumn before continuing with the project.

Feeder twisting

By On · 2 Comments · In Amateur Radio

The open wire feeders are getting rather slack.  This causes the wires to twist together during high winds.  I had noticed a slightly higher SWR and some scratching noises while receiving on the USA Rhombic.

I had to examine the entire run of the antenna feeder and the antenna itself.  Looking up into the bright sky is rather difficult for me due to the condition of my left eye after the accident, but with a good pair of binoculars it only took me about one hour to find and correct the problem.
I guess some maintenance is called for before winter…

Shack Progress

By On · 2 Comments · In Amateur Radio

One operating position is ready – position B which has the control unit for all antennas.   Some work remains, dressing cables, installing computers and building brackets for Bandpass Filters etc.

 

High Power Rhombic direction Switching

By On · Leave a Comment · In Amateur Radio

I have a few of these high power relays, but the relay voltage is unknown and it is not marked.

The relay coil measures 10 ohms.

I intend to use them for Rhombic direction switching.

Measurements with a variac revealed that the relay switches around 60 VAC and is comfortable at 110VAC drawing 0.8A without coil heating.

New Radio Shack progress

By On · Leave a Comment · In Amateur Radio

Most of the cabling work is done. Electric power has been installed, 2x50A via shielded cable and over 20 circuit breakers. The Matrix Strip Line Antenna Switch has been connected, and receive antenna cabling is close to finished. The RX antenna patch panel is installed and connected. 8 receive antenna cables are routed from the patch panel into the Radio Shack and will be connected to a K9AY RAS8x2 receive antenna controller.

cabling
cabling

Equipment room cabling is progressing. The Matrix switch is connected and RX antenna patching is also complete. 2 x 50A Power has also been installed with shielded cabling.

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Rhombic vs Log Periodic at 30M – Shootout

By On · 2 Comments · In Amateur Radio

During my Arctic Diamonds presentation on the CCF/OHDXF Ferry Cruise in January, – Tonno, ES5TV asked me whether I had any ”normal”  antennas to compare my rhombics with.   

My answer was a tongue-in-cheek reply to the effect that since I had unlimited space at my disposal, I wasn´t interested in limited space antennas.

A fellow Rhombic owner who indeed has such a comparison antenna sent me the following audio files which demonstrate the difference between the reception of  ”normal ”  antennas and Rhombics.

The first audio file is of TL0A an Amateur Radio Station located  in the Central African Republic, recorded using a huge Log Periodic at 30 meters height.  

Most Radio Amateurs would kill to have such an antenna ! :-)

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The second recording is made 3 minutes later, from the same location, but this time using a  Rhombic antenna to receive TL0A.

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A visual comparison of the two files  says more than a thousand words:

TL0A on LP antenna

TL0A on LP antenna

 

TL0A received with a Rhombic

TL0A received with a Rhombic

 

My Rhombic colleague commented as follows:

”… you will hear what a 30 meter high 6.5 dB gain antenna sounds like with a very wide radiation pattern. It is ideal to pick up all the noise and electrical storms in the equatorial noise zone.

The other recording is 3 minutes later,  with a Rhombic with 20 dB gain, 10 meters high, low take off angle- and a beam width of just 10 degrees. It is not so much that the signal is that much stronger, it is that the noise is that much less. Note the ease with which it is possible to pick up and follow S1 stations that are very weak.

A Rhombic is not just about the gain, it is much more about the noise which you Dont Get…”

 

I rest my case.


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Tightening Open Wire Feeders

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Due to a variety of reasons, some of my feedlines and terminations are getting slack over time.   I have done some research on this problem and it seems the best way is to use counterweights when installing the lines.

I came up with this temporary solutions to tighten the feeders.   It remains to be seen how well this will perform…

tightening feedlines

This is the termination for the Johannesburg Rhombic (under construction)

feedline tightening

This is the termination for the Johannesburg Rhombic (under construction)

tighening feedlines

This is the feedline of the Tokyo rhombic.   The slack was caused by the center pole twisting, which was in turn caused by the termination being of lesser strength than the other feeders.   I will replace the termination wires with a heavier gauge during the next maintenance and I will then be able to tighten all the feeders equally.

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Stacked Rhombics

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Stacked Rhombics at TF4M.

stacked rhombics

I have finished the task of building a lower Rhombic towards the US.   The lower Rhombic is a two wire design at approximately 50 feet.   It has a fixed termination, whereas the three wire upper rhombic is switchable.

Due to my accident, the task has taken me approximately one year to finish.

The two Rhombics can be phased together with an ArraySolutions StackMatch located near the shack and by switching the upper rhombic it is possible to beam in opposite directions simultaneously.

The total amount of wire in the Stacked Rhombics is 24,300 feet (8100 meters).   This includes the amount of wire consumed by the two 3,000′ long open feeders going out to the site.

Depending on how the stacking works out, I may elect to feed the two rhombics with a 4 wire open feedline (already in place) and a 300 ohm Balun, rather than with two 600 ohm open feeders as is currently the case.  This should give me a few additional dBs in signal strength due to reduced feeder losses.

According to LaPort, the effect of stacking the rhombics is mainly to reduce the number of sidelobes with a more powerful main lobe.  The effect might therefore be a reduction in signal strength for stations off the main beam.

Initial testing is very gratifying, the lower rhombic is even quieter than the upper rhombic,  but I suspect the new antenna will mostly find it’s use in contests where it can be operated independently of the other antenna using bandpass filters, of course.

stacked rhombics - sideview

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Lost Art of Rhombic Antennas

By On · Leave a Comment · In Amateur Radio

I found this scanned article in one of the folders on my hard drive.   I can not remember where I got it from though.

 

Lost Art of Rhombic Antennas

 

The full PDF file may be downloaded from here.

You may download the Foxit PDF Reader here . 

 

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