You Saw where this was headed ~ A 1930's Style Station Part 3

The PMR6A Receiver itself will now undergo the re-incarnation process.

1-17-2020 ~ It is Alive! 

More Space Filler.

Ham Safety while working with Thermatrons (vacuum tubes). 

Most of our homebrewing taking place today uses 12 VDC and that is not high voltage. Although I do know of a case where a constructor had his hand on a 5 VDC 30 amp power supply output -- seems like his ring got shorted across the supply and the ring nearly melted around his finger, which subsequently I think required amputation. So any voltage can be dangerous.

The voltages in the PMR6A under load show 260 VDC on the lead to Pin #3 on the power connector. [There is a clue here for those who read every word...] The problem with the HV is that it can start your heart (defibrillator) or stop your heart if applied too long.

One of the additional steps I will be undertaking is to provide shield covers over the power supply so one does not experience an unwanted jolt! HV kills!

73's

Pete N6QW

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That process will entail a clean up, tube testing, circuit integrity checks and a lot of getting to know the innards of the PMR6A. But that will not happen in one day. So stay tuned.

To have a space filler for this blog since I will be working in the background on the PMR6A Receiver I would like to share a little more about simple rigs that are now nearing over 70 years old!

Enter the WesKit BN-1 Novice Transceiver. You might think of this kit much like the $3.50 Pixie CW Transceivers that are currently on eBay direct from China. Here is a BN-1 in all of its glory!

There are some notable points to observe here and firstly is that it could be operated over the range of 3 to 8 MHz --shades of a Paraset. It was battery operated (a plus for portability) and you could have as high as 180 volts on the plate and as low as 45 volts. Most likely the power output with say 90 VDC on the plate in the less than one watt range. The plate dissipation was one watt and when the two halves are operated in Push Pull the Pout is 2 watts! So definitely QRP. 

Yes a CW transceiver was all done with a single tube, a 3A5 which is a dual triode intended as a power output stage in a battery operated receiver. I did see one variant where a 6SN7 was used in lieu of the 3A5 which opens the door to a bit more power output. But the 6sN7 needs more filament voltage (6.3 Volts)

One slick advertising photo shows the BN-1 with a loaded coil whip antenna on the box but was never produced or sold that way. I can just imagine the number of hams who were getting postal money orders (the plastic card of the 1950's) and sending off to purchase a BN-1. With that whip antenna it was now a complete rig in a box on 80M CW!

With a large tip of the cap to my friend Bill, N2CQR who recently built and operated a homebrew ET-2 that was not unlike the BN-1 in that it had a one transistor regen and an a one transistor 100 milliwatt oscillator. Bill made two dozen contacts in multiple states. So the BN-1 could likely do the same. Most BN-1 field reviews were not so kind attesting to a poor receiver and low power. But others had remarkable records using the BN-1.

Circuit Forensics

I guess one could argue was this a true transceiver as we think of transceivers today? What we have is a tunable regenerative receiver (1/2 the tube) covering the range of 3 to 8 Megahertz and a crystal controlled modified Pierce Oscillator (the other half) covering the same range. So the true transceiver is only when the receiver is near the Crystal operating frequency. But I guess you could operate cross band with the receiver on one band and the transmitter on another. Unlike the Pixie which forces a received frequency near the transmit frequency.

So lets us look a little more closely at the schematics in our tour of the circuit forensics. Although the two schematics are drawn a bit differently they are the same. 

The first thing that catches my eye is the DPDT toggle switch which is the TR switch (on a small metal box). You had to really be deft when you engaged that switch. You guys with 500 pound gorilla hands, if you pounced on that switch you probably would move the received signal location. There are no vernier drives on those tuning capacitors and with a range of 5 Megahertz over the arc of the capacitor --a velvet touch is mandatory. So that could be an issue. 

The only items being switched is filament power between the two separate filaments. However on receive the crystal is grounded but on transmit connected to the plate of the transmit half.; but there is no filament voltage supplied to the transmit half of the 3A5. I have no explanation about why it would be grounded on receive.

However as mentioned on transmit, the crystal is connected to the plate circuit to form a critical function in the kB=1 criteria. The Pierce oscillator uses this topology. BUT normally one sees a blocking capacitor in series with the crystal on the plate side. 

There are many concerns about crystals used for frequency control. Two concerns with the use of quartz crystals are voltage and current. If say you were running 250 VDC on a plate of the 6SN7 variant, then one side of the crystal would be hot with 250  volts without the cap. The other issue is that huge currents can be developed in crystal oscillator circuits and an excessive amount will shatter your one and only rock. 

Perhaps the cap was left out because of cost --a penny here and a penny there. I have no crisp explanation as to why it wasn't installed. 

Now a big caution about Crystal currents. I see there are many FT-243 style crystals being sold on eBay today that are essentially smaller HC-25 or HC-49 crystals stuffed inside a FT-243 holder and re-labeled with a new frequency. Read the fine print... It says these crystals are only good for QRP rigs where the current is low. I suspect you put one of those crystals in a 6AG7 oscillator circuit you will see a mushroom cloud of smoke in your shack

The transmitter tuning involved the old light bulb trick to show when the antenna was taking  juice -- brighter = more fire in the wire! BUT given the link coupling -- this might be an issue with harmonics when operating say on 3706 kHz ( Novice CW frequency in 1950) and the second harmonic would be 7412 kHz. That might get you a post card from an OO. 

So using that rig today might not meet the current FCC spectral purity issues on transmit. A Band Pass filter, should you build this transmitter, is much in order. Today around $7 will get you a tube and socket from Antique Electronic Supply in Phoenix, AZ

Because the regen receiver is in fact an oscillator even in receive it too is radiating a signal. Think about those hams running 10 milliwatts on WSPR and being heard hundreds and even thousands of miles away.

A couple of issues with the receiver! Your Sony Walkman 32 Ohm earbuds will not work as the phones must be high impedance "magnetic" earphones and they are an active element in the receiver plate supply circuit! Also the headphone interconnects are insulated "pin type" connections so another issue. Typically the 3.5 mm stereo jacks have a common ground. Ditto for the Key and Crystal using the same type pin jacks.


Let us now look at the back side of the BN-1 box and again at the schematic.

Since this is the back side the receiver portion is along the left side where clearly we can see Coil L1 and L2. On the right side we see L3 and L4 as well as the connections to the pilot bulb (antenna current indicator) and the two wing nut antenna posts. Pretty clear.

BUT and a BIG BUT ... The schematic does not show any connections from the antenna to the receiver! The antenna is always connected to the transmitter coil L3.

So how does the received get from the antenna to the receiver. Often in a regen receiver you would have a third coil winding that essentially link couples the off the air RF to the tank circuit of the regen. I thought initially if that if coils L3/L4 were close enough to coils L2/L1 then we would have mutual inductance link coupling. But it is clear that they are too far apart for that to happen.

Here is where we can get fooled. I keep looking at that circuit diagram as two tubes. Like Bills ET-2, where there were two separate FET devices, the 3A5 is two triode tubes in a single tube shell. Not two separate tubes!

I am theorizing that in the signal path from the antenna to the regen receiver is the inter-electrode capacitance of the 3A5. Thus within the 3A5 itself the antenna on receive is connected to the receive half through a small capacitance internal to the tube. 

From the NJ7P Tube Data Base we have data for each triode so we can assume there likewise is a small interelectrode capacitance between the two triodes. That value in the 1 to 2 PF range would give "light coupling" between the antenna and the regen. The light coupling is probably good for a smoother regeneration and with moderate band condition some signals would be heard!

Each Triode
Source ........................................ RCA RC-29 - 1975
Input ......................................... 0.9 pf
Output ........................................ 1.0 pf
Grid to Plate ................................. 3.2 pf

[One of our very astute blog reader's emailed me after this was initially posted and made me aware that there was perhaps even a bit more sophistication to this circuit... lurking there in the bushes. If we think of L3 / L4 as the front half of a Band Pass Filter coupled with a very small capacitance (internal to the tube) and in turn that L1 / L2 is the other half of the BPF. This then  would indeed add some front end selectivity. So having the transmitter tank tuned close to the receiver operating frequency makes for a nice way of achieving a BPF and keeping crud out of the receiver circuits. Bravo WesKit!]

Thus today's filler has covered the WesKit BN-1 Novice two band CW Transceiver that likely does work but in itself might not bring thrills but real challenges. 

(The WesKit BN-1 was sold through Western Radio in Kearney, Nebraska and if you mated that rig with a Gotham Vertical --you could work the world!)

73's
Pete N6QW

You Saw where this was headed ~ A 1930's Style Station Part 2

My PMR6A Power Supply Build Progress!

1-15-2020 ~ Final Wiring.

Power OFF -- Neon Pilot Light OFF

Power ON -- Neon Pilot Light ON

The DC Filament Power Supply is Installed.

Plan View of the Power Supply Modules.

The Filament Supply is "Hot".

Work to be done includes final wiring to the terminal strip plus wiring to the Meter and the load tests. The No Load Voltage is + 350 VDC. Another point here is that essentially there is no bleeder resistors other than the radio itself (poor 1950's practice) so there was +350 VDC on one of the pads --for a long time. I will add some protective covering over the areas where there is high voltage.

With the final completion of the power supply I will then turn my attention to the PMR6A. If that is a bust, then I still have a pretty nifty DC power supply for other projects.

73's

Pete N6QW

1-14-2020 ~ More progress on the wiring!

The 12 VDC supply has been shipped and it may be here today. I accomplished more wiring last night but will await the arrival of the DC supply before I install the terminal strips and cable assembly to power the PMR6A. (A board space issue.)

A couple of notes here. I am using a 1.5 Amp fast blow fuse in the cartridge and once I get things working probably will shift that to a one amp. I intend to form the capacitors by using a variac on the primary side at the first "power on" cycle. Once at full power I will let it burn in for maybe an hour. 

We will not connect the PMR6A initially and to substitute a load will install a power resistor on the main HV lead where the VDC = 260 and the Amps = 0.1. So applying E=IR, R value is 2600 Ohms  and the power = I^2*R or 26 Watts. So If I take four 10K , 10 watt resistors in parallel that will give me 2.5 K at 40 watts. You got to love the math!

BTW the Panel Meter will assist in discovering the No Load and Full Load Voltage of the HV supply. It will also provide a bench mark of future power cycles as it should not vary significantly from the Full Load Condition once in operation. An important piece of troubleshooting info downstream.

Now I do not know the actual condition of the PMR6A and so that is where dusting off my Sencore Might Mite Tube Tester will come into play. That also means powering up that old laptop as that is where the tube data for the tester is now hiding. 

Now an old TKT (tribal knowledge tip). Do not pull all of the tubes out at once and throw them in a pile -- you absolutely will not remember what tube goes where. Do it one tube at a time and perform another important step. 

This additional step is where I have a small plastic tub and I spray a bit of De-Oxit into the tub and each of the tube pins is dipped in the tub and are cleaned off before being put in the tube tester. So when each tube is plugged back in -- they are clean pins -- I also give them a quick spray with De-Oxit after testing so now the tube sockets in the PRM6A are cleaned up. RESIST spraying De-Oxit into the tube sockets!

Also I plan that at power on after the tube testing will be the filaments only. If all the tubes light in the tester and the tubes test good then if there are some that do not light once back into the radio -- either I have a wiring issue in the PMR6A or managed to mess up the tube when being put back in the radio.

BTW I also verify the tube number and match that to the tube location in the manual. Some prior owner may have put the right tube in the wrong socket. This is another reason -- DO NOT power ON the RADIO until you test the tubes and assure the right tube is in the right socket! Also note on the schematic that one of two different tubes were used at V2. It is all a process guys! 

Ditto for the wiring -- ugly construction is not a good idea when using HV!! My wiring while not Mil-Spec looks good and also facilitates trouble shooting as you can trace the wires and it is not a jumbled mess! When you know stuff you can do stuff!

1-13-2020 ~ A few changes and a DC Supply

A bit more power supply wiring and a few changes. Initially I had a very small DPDT mini-toggle switch for the main power switch -- it had been liberated from another project. As I was removing the wiring I noted that one of the terminals "wiggled" in the housing. No need to add problems. That is when I switched to a full size toggle switch.

When I bought parts for the supply several years ago, that buy included a 600 Ohm 10 watt resistor. Somehow that resistor got put in a bag with other components and seemed forever lost in my huge junkbox. I substituted a 600 Ohm at 5 watts thinking that would need replacement. While looking for some quick connect terminals for the neon pilot light -- boom there it was and so the 5 watt was removed and the 10 watt installed.

Clever blog viewers will spot a dangling black wire. That will be properly terminated (shrink wrap) and tucked under the transformer housing. Hammond transformers come with a tapped primary for either 117 or 125 VAC. I used the 117VAC and so the black wire which is used for the 125 VAC tap is redundant. There are a couple of more wires to be added today including the power switch, a jumper on the PC Board and adding two additional wires to the transformer primary circuit where 117 VAC is supplied to another module. 

Yet to be done is the wiring for the two terminal strips that will house all of the wiring that will be fed to the PMR6A via a three foot long cable and an 8 pin Socket with shell. That cable plugs into the rear apron of the PMR6A. The other open item is installing on the board and wiring of the cool (and small) panel meter.

The filament problem is resolved. All Electronics (here in Southern California) sells a 12 VDC supply at 5.6 Amps for under $10. That supply will be mounted vertically (smaller footprint) on the open space to the right of the PC Board. BTW All Electronics has flat rate shipping --so I bought some other needed supplies. They have a website!

The delivery of the DC supply will not be until Tuesday so we are a couple of days away from having a fully operational PMR6A  power supply. I also suspect that having a DC filament supply will help a bit with AC hum so often heard a low volume levels on "Toob" radios.

1-12-2020 ~ Wiring the Power Supply!

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Dabbling with our hobby would be no fun unless we had a few problems that actually cause us to grow and learn. In the quest to build a 1930's station I started by actually trying to finish something initiated several years ago and that is to get on the air a Multi ELMAC PMR6A Receiver. That in turn would cause me to build a suitable power supply first, as I had most of the components (bought at the time I picked up the receiver) but these were never assembled. 

So I have "noodled" several approaches and settled on building a master circuit board which would be mounted on a wooden base. Initially I have specified one size of base and then in a stroke of luck picked up a piece of "cut-off" lumber at Home Depot which I cut in two. The other half --yes will be for the transmitter! The size now for both boards is now 8.75 X 9.25 inches by 7/8 inch deep. This is bigger than my initial approach; but has proven to be a good thing because of an issue I encountered. 

But first some progress photos.

The above photos show how the PC Board was cut on my CNC and then mounted on the wooden board. To the right of the board is open space that will be used to solve one of the problems you will read about further on in this post.

The PMR6A has the ability to turn on the power supply but I will not use that feature; but instead use a local mini-toggle switch. Next to that switch (rectangle with a red inside rectangle) is a 115 VAC neon pilot light. There will be no mistaking that the supply is hot! 

On the back side is the IERC three pin power jack and a fuse so that we have the PMR6A meet the current electrical standards. Something to be added on the front panel at the middle  is a cool (and small) 0 to 500 VDC panel meter. That along with the neon pilot light will certainly add to the 1930's look.

A construction note here. While at Home Depot I picked up some 3/8 inch nylon spacers and 1 inch long wood screws. The power transformer has been elevated above the wooden base by 3/8 inch. Why? Wire management. I will have to run wires to the power jack/fuse and to the 115 VAC neon pilot light and power switch.  I also do not intend to cut the power transformer wires. Thus this space under the transformer promotes wire routing and management as well as providing a space to bundle up excess wires. When you know stuff, you can do stuff! 

BTW having both a CNC and manual milling machine sure makes cutting rectangular and circular holes in various materials a lot easier than the old drill holes and use the file method.

The power supply schematic should have been a clue that there was a problem brewing! But I missed it.

Many of the PMR6A's are marked on the back cover 6V or 12 V and this bypassed me. My PMR6A has no marking (as I discovered). The clue in the power supply diagram is at the very top of the power transformer that shows a switch for 6V or 12V. The actual Schematic in the manual I downloaded shows two wiring configurations for either the 6V or 12V. 

The first problem is that my power supply transformer only has a 6.3 VAC winding at 3.5 amps so while that would be OK for the 6V version --a bit short for the 12V version.

I popped the hood (actually it was the bottom cover) and after a bit of tracing in the rats nest of wires -- mine is the 12V version.

So we are left with a few alternatives for the filment supply:

1. Rewire the filaments so that all filaments are in parallel and thus would operate from the 6.3 VAC winding. Easier said than done because of the tight space. Doable but not without a lot of hair pulling and swearing. Not totally discounted at this point and involves no additional cost.
2. So OK take the 6.3 VAC and build a voltage doubler supply and then run it through a voltage regulator so that you get 12 VDC. (The PMR6A filaments can run on either AC or DC). A possibility … but more hardware to build and we are talking amps so big power pass transistors.
3. Just bite the bullet and purchase a 12 VAC filament transformer say at about 3 amps and install it on the board. Certainly a pretty simple hookup but does require some added board space. Hmmm TRGHS in that now the wooden base has a larger foot print.
4. Purchase a 12 VDC 3.5 amp power supply and install it on the board. Such supplies tend to be pretty small and should fit on the board. Now we have to evaluate the cost of the supply versus the cost of a transformer.
5. In looking at items 3 and 4 -- a 12 VAC transformer at 3 or 4 amps is roughly in the $13 to $15 range. I did find a 12 VDC 3.5 amp DC supply for about $8. Still noodling this one but will make the decision today. I can finish the build of the HV part of the supply but can not power on my PMR6A until I get the filament supply whacked!

Today was the first day I actually looked inside  my PMR6A and it is not unlike the photos that I used in the earlier post. A bit dirty -- and it contains many "dog bone" style caps -- no brown colored resistors ( which may be evidence of overheating). Keep in mind this is a journey that we started by building the power supply. A smoke test is in the very near future! 

The garage right now is about 39F -- "a bit nippy" said the topless stripper as she rode the metal pole!

73's

Pete N6QW

You Saw where this was headed ~ A 1930's Style Station

What if you wanted to build a vintage 1930's Station?

1/10/2020 ~ Some economic perspectives from 1933 as that will frame what hams could spend for a rig in comparison to meeting their daily needs. [This is just a quick Internet search; but is an eye opener.]

Some basic 1933 survival expenditures:

• A Loaf of Bread was 7 Cents ($2.79)
• A Can of Soup was 10 Cents ($1.99)
• A Gallon of Gas was 10 Cents ($4+)
• A Glass of Beer was 10 cents ($5.00)

The values in () are some local prices as of today. Thus pennies in 1933 could purchase some vitally needed staples, which were in direct competition with buying radio parts. 

I found it interesting that a can of Campbell's Cream of Mushroom Soup has fared the best in the last 90 years. BTW at one time I worked on a project for Campbell Soup counting cans using electronic means versus mechanical counters. At that time (1980's) about 90% of the cost of a can of soup was in the can not the soup. At their Sacramento (CA) plant, Campbell manufactured their own cans so with such a high cost in just the can manufacturing costs were tightly controlled. 

Regrettably Beer has fared the worst!

Certainly the daily economics forced many a ham to be creative and to use common items in their radio projects. The term "to breadboard a circuit" has its roots in using the kitchen breadboard as a chassis. Pie tins also found their way into radio projects. Hams in 1933 had to "know stuff" and that extended beyond using their smart phone and credit card to buy electronic hardware on Amazon or eBay!

Imagine telling a ham in 1933 he could buy an entry level ICOM 7300 for around $1000 when a beer was 10 cents?

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Building a 1930's style station ?...

Well obviously most responses would be: Say What? Who in the world would want to do that when I could buy a uBitx v6.0 with the Nextion display and all of those bands and a whole cadre of supporters and technologist tinkerers who daily post to the Bitx20.io.

The answer is because you can! But this is not for 90% of those who may stumble across this blog. So time to tune out.

But for the very small percentage (maybe less than 10%) who may want to dabble with the technology of old we are about to start an amazing journey. 

Disclaimer -- I may use some 2020 technology to solve a problem where parts may just not be available but no Si5351's or Arduino's are involved. (There Bill!)

I have been doing some research on 1930's parts and am surprised that they can be found and in some cases really cheap. 

Transceivers as we know them today were yet to arrive; but there were a few that were documented. 

For the most part the 1930's vintage station had a receiver, a transmitter and some form of antenna change over. I laugh as my first antenna change over circa 1959, was a Double Pole Double Throw Knife Switch. You really had to be careful when you made the TR to keep your hands off of the metal part. More than once I sustained RF Burns --- it is clear when that happens as your skin turns purple at the contact area!

Several years ago I purchased from a local SK sale an amazing receiver that was the ICOM 7300 of 1953. The receiver is the Multi Elmac PMR6A. On eBay right now is one for sale for $29. A bit rough but restorable.

The one I picked up was in really good shape but no power supply. This may be cheating a bit on the receiver but now is the time to build that power supply --which could be used on other receiver projects.

The PMR6A was really a portable, mobile or home station receiver. The one in the photo above is a slightly later version than mine as there have been mods made to upgrade this to a PMR7. Basically this a AM/CW rig (it does have a tunable BFO) and even a noise blanker (well noise limiter). It covered the five major ham bands (80-10) and as a bonus AM and 160M. Basically it is beyond a regen and chosen because it was in my storage bin.

In case you haven't spotted it --the IF is at 455 kHz. Given it has a tunable BFO, what if you were able to squeeze a Collins Mechanical Filter into that circuit? Boom an SSB capable receiver --lots of possibilities and just waiting for further exploration. Will have to add a real product detector in lieu of the 6AL5 Diode Detector  -- but a socket is there and an opportunity for another circuit modification. 

Maybe a 6AK5 as a PD candidate but I would have to make adjustments for the filament current-- the 6AL5 uses 0.3 amp and the 6AK5, 0.175 Amp so you would need to have a resistor in parallel with the 6AK5 filaments to absorb the additional current flow. Thusly E = 6.3 and I = 0.125 (.3 -.175) which results in a R of 6.3/.125 = 50.4 Ohms and the wattage is I^2*R or less than 1 watt so use a 1 watt 50 Ohm resistor and Bob's your Uncle!

The very 1st thing that should catch your eye is that this is basically equivalent of a sardine can with lots of product stuffed into a very small space. It was not designed with DFMA in mind.

I do not know if my unit even works --but some of those caps will most likely need a retrofit. I will bring up the voltages slowly using a variac to try to reform the electrolytic caps. But the 1st task is to down load the manual from the BAMA Edebris site. You absolutely need the manual or you will be lost!

The next task is to build a fixed station power supply and below are the schematics for the several possible supply configurations. The AC supply is on the right hand side of the schematic.

I chose a Hammond Transformer that would deliver 250 VDC at 100 Ma. and the Choke also a Hammond is 10 Henry also at 100 Ma. I will use diode rectifiers in lieu of the 6X4 rectifier.

Initially I even bought a chassis to house the supply; but then decided I would have to invest in a lot of metal bashing. Now mind you it has been some time since I used my Greenlee chassis punches but I do have them.

So my feeble brain said: "what if you could build it another way", and that is when I came up with the idea of using a piece of 6"X12"X1" pine board and make a circuit board that would house most of the components and that circuit board would then be simply screwed down to the pine board.

So using my tried and true method of laying out the actual pads using graph paper I was able to develop coordinates so I could mill out the board. This same process could be used to etch a board or even use a Dremel tool to create the pads. When I say circuit board it is not what you think...

This uses a stock size of 4X6 inch single sided copper PCB. The finished board looks like this. The 10 Hy choke will mount right on this board in the open space in the middle of the board.

Be it known that I was born at night but not last night. Should I chose later to install this power supply in a chassis, the circuit board is easily retrofitted inside the underside of the chassis and would involve very little metal bashing. (When you know stuff...)
 
I am awaiting some power supply components like the HV electrolytic caps from Antique Electronic Supply( $7 gets you the three caps -- I used 20 Ufd at 450 VDC for all three). These won't be here until Friday.

The next several rounds of blog posts will focus on the building of the supply. I did buy most of the components (transformers, chokes, etc.) at the time I bought the PMR6A so it is now time to get it working.

Once the supply is working we will start the process of bringing the receiver back to life.

The other half of this combo in the 1950's was the Elmac AF-67 transciter. I have one of those too -- but that was a gift and not purchased -- that gift is what prompted the PMR6A purchase.

Those that know stuff,can do stuff!

Pete N6QW
 
Spoiler Alert -- you will be surprised at the transmitter build -- It is a real departure for me. Recently I was put on to a vintage radio company that was moving away from this product line. They did include spark gaps as one of their featured products. Think Plate Modulated Tesla Coils and other off the wall ways of generating RF.