Wednesday, November 6, 2013

Remote selection relay design

We recently had an application where we needed to remotely switch between two RF amplifiers.  I used Google and searched to find a solution to no avail.  So I am posting this to save the next person a lot of time.

All we had available was the single transmission line coming from the RF amplifier.  We were sending DC up the transmission line to power the amplifier.  Because of a lightning issue we could not use anything “solid state” to create a working solution.  The first thing that comes to mind is reversing the 12 VDC to switch amplifiers, but then the amplifiers negative input was fastened to ground so that cannot be done. 

However there are some solutions to switching between the two amplifiers by pulsing the power to the remote amplifier.  All we can do is interrupt the power for a fraction of a second to switch between the amplifiers.

One solution is to use an “alternating” “impulse” or “bistable” relay.  The most common such relay is the S89R11DAC1-12. It is available from all major vendors.   The biggest problem with that relay is that it draws half an amp of power.   That might be solved by putting a capacitor between the power source and the relay so that the relay is only activated for a few seconds when the power is turned on or when it is turned off then back on.  It is also a rather large relay and it is not socketed for easy replacement.  Here is a picture of what that relay looks like.

Solution number two is to use what is called a magnetic latching relay.  These relays have two coils, one to turn it on and one to turn it off.  A capacitor in series with the relay creates a pulse that either turns the relay on or turns the relay off depending on what coil is selected.  There are several kinds of these magnetic latching relays that are available.  Model number 755XBXCD-12D has a round base and 785XBXCD-12D has a square base.  Here is a possible, untested, schematic.

The next solution is to use a conventional relay but use a capacitor to detect how long the power was off.  If the power has been off for a long time then the capacitor will be discharged.  When power comes back on the capacitor is a “short” thus delivering 12 volts to the relay.  If the power was only off for less than a second, then the capacitor is still charged and the relay only gets about three volts.  That is not enough voltage to turn the relay on.  The 12 volt relay I tested had a 150 ohm coil and required 9 volts to turn on, but it will stay on all the way down to three volts.  Hence the relay will be turned on or off depending on how long the power was off.  Here is the schematic; it was tested to work with a power supply of 9 volts all the way up to 16 volts:

Friday, November 1, 2013

A brief history of health insurance - why obamacare is a failure

A brief history of health insurance – or why some of us knew that obamacare would be a flop.

**This is my personal opinion and is full of sarcasm, it is not meant to be entirely accurate**

Many years ago, most people did not have insurance.  However, back then there were lots of “nonprofit” hospitals that would gladly take anyone without insurance.  When my first two children were born we paid the hospital $100 a month for 10 months to cover all of the expenses.  The money went directly to the hospital or doctors, there were no middlemen.  Eventually those “nonprofit” hospitals were bought out and bulldozed.  Hospitals are now in the business of making money.

Then after that change, there was what is called “Major medical” insurance.  It cost our employer about $1000 a year and it covered everything medical that totaled over $2000.  We kept all of the paperwork ourselves and then submitted it at the end of the year.  Our insurance company then sent us a check for everything that totaled over $2000.  We then used that money to cover the medical expenses for the next year. 

Then disaster struck.  Our wonderful government thought that it would fix the “problems” with health insurance.  They passed laws that invented what is called an “HMO”.  HMO insurance included “Free” physicals and many other “free” things because, as we all well know, if you see the doctor for free you will not get sick and hence you will save a lot of money.  HMO’s also required that the primary care doctor would do all of the paperwork so he had to hire two or three secretaries just to process all of that paperwork.  HMO’s would save money because the doctor would detect problems earlier and you would not get as sick.  OK, maybe the premiums would go up a little because someone had to pay for all of the secretaries to process the paperwork and someone had to pay for all of those “free” physicals, etc.  Insurance went from $1000 a year to $10,000 a year.  Thanks to the US government for figuring out how to save us money.  As a result many people were no longer insured.

Then along comes more help from the “experts” on saving money.   Yes ObamaScare to the rescue!  His idea was that if those who were well paid a little [lot] more they could cover the expenses of those who were sick.  He also eliminated being refused insurance because of “preexisting” conditions.  He also included coverage for a lot of things like abortions and sex changes.  Of course someone has to pay for all of those new extra benefits.  On top of that, if you had insurance that did not include all of the new extra benefits, it has to be cancelled.  So as a result about 1/2 to 3/4 of those with insurance will find that their insurance is now cancelled.  However they will not be able to afford the increased premiums, to pay for those that are already sick, so in the end as much as 1/2 or more of the population will be left without any health insurance.

On top of that there is the now delayed “Employer Mandate”.  Any company with more than a few employees is required to give “free” health care insurance to all of its full time employees.  The solution is simple, make them “part time” employees.  This led to massive job reductions and that led the “Employer Mandate” being delayed until after the election.    

Now if we were to go back to directly paying the doctors we could cut the cost in half because all middle men (insurance companies) do is increase the cost of seeing your doctor.  Remember ObamaScare is not medical coverage, it is health insurance, there is a huge difference there.

Common Radio Frequency Questions - RF FAQ

Radio Frequency FAQ’s
I now work for a company called ComProd Communications.  You can find us at    We specialize in Creating RF Solutions.  There are several questions that we get asked a lot.  Here are some of the most common questions.

My antenna came with a ground plane; do I need to use it?
The ground plane is a critical part of any antenna system.  It has both a minimum size as in length and width, as well as a specific distance that it has to be from the antenna.  If you have to replace it for some reason the replacement but be larger in size as far as length and width, and it must be the exact same distance from the antenna!  The distance is critical because the reflected signal has to be in phase with the signal emitted from the antenna.  Changing the distance will cause canceling of the output of the antenna.  Making it smaller will affect the impedance of the antenna as the amount of reflected signal will vary over the length of the antenna.

Why has the cost of Combiners gone up while the cost of transmitters and receivers has gone down?
As transmitters and receivers have gotten cheaper their sensitivity and selectivity has gone down.  If the combiner does not remove unwanted frequencies then the receivers will be overloaded in their front end.  Let’s say we have a transistor RF amplifier running on 12 volts.  If it has an input signal that produces an output that is over 12 volts peak to peak it will turn it into a square wave and produce tons of harmonics.  This will totally hide the frequency that you want to receive.  The combiner must remove the unwanted frequency from both the transmitter and the receiver in order for the receiver to work correctly. 

Are antennas best stacked vertically or horizontally?
Back in the early days of audio it was common to see lots of speakers positioned horizontally across the front of the stage.  The concept was to have a speaker aimed at everyone in the audience.   At some point engineers started pointing out that if you stack the speakers one on top of the other (Vertically) you get much better sound dispersion.  Although this does not look correct to the eyes, time and experience has proven that it works.  Now most bands use vertically stacked speakers.  The same is true with antennas.  Mounting antennas one above the other seems like it would be sending signals out into space, but instead it concentrates the beam horizontally.

Why do we need separate receive and transmit antennas?
Transmitters emit some signals at unwanted frequencies, usually close to their primary broadcast frequency.  These unwanted frequencies are rapidly reduced in strength as you move away from the transmitting antenna.  However these unwanted signals can easily interfere with receivers that are operating on nearby frequencies.  If you have a separate transmit and receive antenna this problem can be drastically reduced.  Otherwise the frequency of the receiver needs to be removed from the transmitters output as well as the frequency of the transmitter must be removed from the receivers input.