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Transmitting with no antenna?

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donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Mon Jul 21, 2014 12:27 pm Post subject: Transmitting with no antenna?

I'm working on an audio panel/radio problem and I was told that pressing the PTT (transmitting) without an antenna will fry the radio.   



Is this correct or is it an OWT?

henry(at)pericynthion.org
Guest

Posted: Mon Jul 21, 2014 1:26 pm Post subject: Transmitting with no antenna?

It's not a good idea; transmitting into an open circuit can damage

that final power amplifier stage as the signal is reflected back into

it.  But most radios built in the past 20-30 years will have built-in

foldback protection to detect this condition and dial the power back

to avoid damage.



Henry



On Mon, Jul 21, 2014 at 1:27 PM, donjohnston <don(at)velocity-xl.com> wrote:

 	  Quote: 	
		   



 I'm working on an audio panel/radio problem and I was told that pressing the PTT (transmitting) without an antenna will fry the radio.  [Embarassed]



 Is this correct or is it an OWT?





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p=427077#427077

donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Mon Jul 21, 2014 1:50 pm Post subject: Re: Transmitting with no antenna?

 	  henry(at)pericynthion.org wrote: 	
		  It's not a good idea; transmitting into an open circuit can damage

that final power amplifier stage as the signal is reflected back into

it.  But most radios built in the past 20-30 years will have built-in

foldback protection to detect this condition and dial the power back

to avoid damage.



Henry



On Mon, Jul 21, 2014 at 1:27 PM, donjohnston <don> wrote:

	
 



Thanks!



What should I do to prevent any potential problem?  Would a 50 ohm terminating resistor be a good idea?  I'm not going to be able to connect the antennas for a while and I'd like to not have to worry about any problems.

JOHN TIPTON

Joined: 17 Sep 2006
Posts: 239
Location: Torquay - England

Posted: Mon Jul 21, 2014 1:54 pm Post subject: Transmitting with no antenna?

Is it an OWT (old wives tale): I don't know but where are those 'ouput watts' going, if you don't have an aerial connected



John



Sent from my iPad



     ----x--O--x----



 	  Quote: 	
		   On 21 Jul 2014, at 09:27 pm, "donjohnston" <don(at)velocity-xl.com> wrote:

 

 

 

 I'm working on an audio panel/radio problem and I was told that pressing the PTT (transmitting) without an antenna will fry the radio.  [Embarassed] 

 

 Is this correct or is it an OWT?

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=427077#427077

billp(at)wwpc.com
Guest

Posted: Mon Jul 21, 2014 1:59 pm Post subject: Transmitting with no antenna?

It depends on the radio design. The problem is that the radio is meant 

to have an antenna connected to it that is designed to operate on the 

frequency the transmitter is transmitting at. When that doesn't happen 

all the power that the transmitter is producing is "reflected" back into 

the transmitter. That can fry things. Most well designed modern 

transmitters have protection circuits that reduce the transmitter power 

or disable the transmitter when it detects that condition. It's still a 

bad idea to transmit with no antenna (or the wrong antenna) connected. 

This goes for Comm radios, Transponders, Radio Altimeters, TCAS boxes...



Bill



On 7/21/14, 1:27 PM, donjohnston wrote:

 	  Quote: 	
		   



 I'm working on an audio panel/radio problem and I was told that pressing the PTT (transmitting) without an antenna will fry the radio.  [Embarassed]



 Is this correct or is it an OWT?





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p=427077#427077

billp(at)wwpc.com
Guest

Posted: Mon Jul 21, 2014 2:22 pm Post subject: Transmitting with no antenna?

Most 50 Ohm terminating resistors are rated for a half watt or less. 

Most Comm radios put out 5 watts or more. The terminating resistor won't 

last long. 1) Keep the radio turned off when you're working on things. 

2) Disconnect the PTT wire from the radio connector if that's feasible.

Termination resistors are made to keep a constant impedance connection 

on something like a spare antenna diplexer port, not dissipate the power 

from a transmitter. You can get 50 dummy loads that are designed to be 

transmitted into all day long. They're a lot more expensive than a 

terminator and much bulkier. Even a dummy load will only operate at it's 

rated power for a limited time. Make sure you look at all the specs for 

what you buy. Most of the time you're going to be transmitting without 

you knowing it because a wiring short or sitting on the mic (or 

something like that). You're likely to be transmitting for a long time 

or until the stuck mic circuit in the transmitter shuts it down.

Bill

On 7/21/14, 2:50 PM, donjohnston wrote:

 	  Quote: 	

 henry(at)pericynthion.org wrote:

> It's not a good idea; transmitting into an open circuit can damage

> that final power amplifier stage as the signal is reflected back into

> it.  But most radios built in the past 20-30 years will have built-in

> foldback protection to detect this condition and dial the power back

> to avoid damage.

>

> Henry

>

> On Mon, Jul 21, 2014 at 1:27 PM, donjohnston  wrote:

>

 Thanks!

 What should I do to prevent any potential problem?  Would a 50 ohm terminating resistor be a good idea?  I'm not going to be able to connect the antennas for a while and I'd like to not have to worry about any problems.

 Read this topic online here:

 http://forums.matronics.com/viewtopic.php?p=427085#427085

john.maccallum(at)bigpond
Guest

Posted: Mon Jul 21, 2014 2:23 pm Post subject: Transmitting with no antenna?

As per some other post in reply yes it can lead to damage to the output

stage of the connected radio. The high SWR presented to the

Output side by no Antenna or the wrong antenna causes significant heating to

the output transistors and can cause their internal junctions to fail.



You can go to your local Ham Radio shop and purchase a small 50 Ohm dummy

load for a few dollars to protect the connected radios or indeed test the 

Radios without an antenna. You will need a VHF, 50 Ohm Dummy Load Capable of

handling 25 watts (less if you keep the transmissions short say 4-5

seconds).



Cheers

John MacCallum

VH-DUU

RV10 41016 





--

edpav8r(at)yahoo.com
Guest

Posted: Mon Jul 21, 2014 5:10 pm Post subject: Transmitting with no antenna?

Alternatively, you could build this dummy load: http://bit.ly/1wSMtI1



..using 20 of these (buy 25 for the price break): http://bit.ly/1lnVgfM



Note: Do *not* substitute wire-wound resistors!



Eric

 	  Quote: 	
		   On Jul 21, 2014, at 5:22 PM, John MacCallum <john.maccallum(at)bigpond.com> wrote:

 

 You can go to your local Ham Radio shop and purchase a small 50 Ohm dummy load for a few dollars to protect the connected radios or indeed test the Radios without an antenna. You will need a VHF, 50 Ohm Dummy Load Capable of handling 25 watts (less if you keep the transmissions short say 4-5 seconds).

 

 Cheers

 John MacCallum

 VH-DUU

 RV10 41016 

 

 

 

 I'm working on an audio panel/radio problem and I was told that pressing the

 PTT (transmitting) without an antenna will fry the radio.  [Embarassed] 

 

 Is this correct or is it an OWT?

nuckolls.bob(at)aeroelect
Guest

Posted: Mon Jul 21, 2014 5:46 pm Post subject: Transmitting with no antenna?

At 03:27 PM 7/21/2014, you wrote:




I'm working on an audio panel/radio problem and I was told that 

pressing the PTT (transmitting) without an antenna will fry the 

radio.  [Embarassed]



Is this correct or is it an OWT?

    I wouldn't make a habit of it . . . there's no

    good reason to do it. If you need to listen to

    your transmitted signal without radiating it more

    than a few dozen yards, build yourself a dummy

    load to put on the transmitter's coax connector.



http://tinyurl.com/cchp3pf



    This load is good for 2W continuous and will handle

    10W transmitters for the few seconds necessary to make

    a vocal test transmission to deduce audio quality.

    If you need a bigger load, it too can be fabricated

    from R-S parts.



    But the caveat about keying a transmitter into

    an open antenna jack is a carry-over from the state

    of solid state radios of the 60-70's. It didn't

    take the industry long to figure out ways to build

    protections into their products . . . so keying

    an unloaded transmitter no longer represents

    a serious hazard.

   Bob . . .

nuckolls.bob(at)aeroelect
Guest

Posted: Tue Jul 22, 2014 7:46 am Post subject: Transmitting with no antenna?

At 08:43 PM 7/21/20, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com>



         At 03:27 PM 7/21/2014, you wrote:

 --> AeroElectric-List message posted by: "donjohnston" <don(at)velocity-xl.com>



 I'm working on an audio panel/radio problem and I was told that pressing the PTT (transmitting) without an antenna will fry the radio.  [Embarassed]



 Is this correct or is it an OWT? 	




   After crafting my reply last night I was curious as to

   modern approaches to crafting SWR ruggedness into a

   VHF transmitter. 



   The last time I dug through the innards of a solid state

   vhf transmitter, the output stage was crafted from NPN

   silicon junction transistors . . . about 4 of them in 

   push-pull for 100W of output at 145MHz. The antenna

   matching network was fitted with a directional coupler

   (SWR meter) that sampled reverse-voltage as a product of

   high SWR . . . that voltage was used to reduce drive

   to the output stage for the purpose of protecting the

   transistors should the transmitter be presented with

   a high swr. Works good, lasts a long time.



   Just for grins, I got a copy of the KX165 transceiver

   service manual to bone up on contemporary design

   philosophies. Got an interesting surprise.  



   Here is an excerpt from the transmitter schematic . . .

   Yup . . . there it is . . . the expected directional

   coupler. But when I read the accompanying text, I was

   informed that the directional coupler output was

   used to sample FORWARD power for the purpose of leveling

   the transmitter's output power. No mention was made

   for 'high SWR protection' . . .



   In fact, a search of the entire service manual

   for "standing" or "SWR" produced no hits. Say

   what????



   

  [img]cid:.0[/img]



   Here are some excerpts from the manufacturer's data sheets

   on the KX165 powuer output transistor. 



 [img]cid:.0[/img]

  This is a MOSFET rated at 80W of output in VHF operation.

  Further we read the following . . .



 [img]cid:.1[/img]



  Down in the tabulation of characteristics we also

  find . . .



 [img]cid:.2[/img]



  I was pleased to find that my original assertion was

  correct but for the wrong reasons. It seems the MOSFET is

  immune to damage from the stresses that would have

  shot its ancestors out of the saddle. Hence no wrap-around

  protection was needed. The directional coupler was still

  there but pressed into service other than output

  stage protection.

   

  Another interesting feature from the schematic excerpt:

  Notice the "stub" depiction on the antenna output line.

  A search of the manual doesn't speak to its existence

  or purpose. Given that this is a deign from the hallowed

  halls of the Nav/Comm gods, I suspect that 'stub' is an

  shunt filter element tuned to reject transmitter harmonics

  that fall on or close to GPS frequencies. Garmin used to

  offer a 330-00067-00 Notch Filter intended to be 'scabbed

  on' to the nav/comm antenna connector.



 [img]cid:.3[/img]



 

 

   Bob . . .

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donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Tue Jul 22, 2014 3:42 pm Post subject: Re: Transmitting with no antenna?

 	  nuckolls.bob(at)aeroelect wrote: 	
		  At 03:27 PM 7/21/2014, you wrote:

I'm working on an audio panel/radio problem and I was told that 

pressing the PTT (transmitting) without an antenna will fry the 

radio.  [Embarassed]



Is this correct or is it an OWT?

    I wouldn't make a habit of it . . . there's no

    good reason to do it. If you need to listen to

    your transmitted signal without radiating it more

    than a few dozen yards, build yourself a dummy

    load to put on the transmitter's coax connector.



http://tinyurl.com/cchp3pf



    This load is good for 2W continuous and will handle

    10W transmitters for the few seconds necessary to make

    a vocal test transmission to deduce audio quality.

    If you need a bigger load, it too can be fabricated

    from R-S parts.



    But the caveat about keying a transmitter into

    an open antenna jack is a carry-over from the state

    of solid state radios of the 60-70's. It didn't

    take the industry long to figure out ways to build

    protections into their products . . . so keying

    an unloaded transmitter no longer represents

    a serious hazard.

   Bob . . . 	




Using the logic of "if some is good, more must be better", I picked up some 10w, 100ohm resistors.    



Checked with the manufacture (Val) and they said that I'm probably okay:



 	  Quote: 	
		  In regards to circuit protection, the COM 2KR doesn't have any active protection against keying the unit without a proper load attached.  As such, you run a small risk of damaging your radios keying them up without an antenna.  However, the radios are resistant to damage from being keyed without an antenna and should be fine.

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edpav8r(at)yahoo.com
Guest

Posted: Tue Jul 22, 2014 4:28 pm Post subject: Transmitting with no antenna?

Don,



Just be sure the resistors you sourced are not wire-wound types.  At a 10W rating, most are.  These are inductive and will make a very poor dummy load.



Examples to avoid:  http://bit.ly/1A3zd7U  -or-  http://bit.ly/1rImumj



Bob's 2W-10W dummy load appears to be built with Metal Film or Metal Oxide resistors.



Eric

 	  Quote: 	
		   On Jul 22, 2014, at 7:42 PM, "donjohnston" <don(at)velocity-xl.com> wrote:

 

 Using the logic of "if some is good, more must be better", I picked up some 10w, 100ohm resistors.

donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Wed Jul 23, 2014 6:05 am Post subject: Re: Transmitting with no antenna?

 	  edpav8r(at)yahoo.com wrote: 	
		  Don,



Just be sure the resistors you sourced are not wire-wound types.  At a 10W rating, most are.  These are inductive and will make a very poor dummy load.



Examples to avoid:  http://bit.ly/1A3zd7U  -or-  http://bit.ly/1rImumj



Bob's 2W-10W dummy load appears to be built with Metal Film or Metal Oxide resistors.



Eric

 	  Quote: 	
		   On Jul 22, 2014, at 7:42 PM, "donjohnston" <don> wrote:

 

 Using the logic of "if some is good, more must be better", I picked up some 10w, 100ohm resistors.   



	
	




Yep... They're wire wound.  



I can't get anything right lately.

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Jul 23, 2014 6:54 am Post subject: Transmitting with no antenna?

 	  Quote: 	
		  Yep... They're wire wound.



I can't get anything right lately.  


	


I was skeptical and disappointed when your conversation

with VAL suggested that 'your were probably going to be

okay' with the 10W resistors.



It would have been nice if the design I posted was

a little beefier . . . but getting resistors

with low inductance at VHF frequencies isn't a trivial

task. The venerable carbon composition resistor popular

to this legacy DIY task are not easy to find and not cheap

when you do find them. The 2W design proved to be the

best I could do with limited resources . . . although

suited to the task as long as you don't get into long-winded

test transmissions. As I recall, I produced that suggestion

as a test tool for proving that some appliance sensitivity

to transmitters was a function of energy radiated from

the antenna.  Short transmissions would suffice.



If you want to acquire a more robust load, finding

suitable resistors then stacking fab-time

on top of the project is problematic. Suggest

you consider commercial-off-the-shelf products

like this:



http://tinyurl.com/k5w4vth



You'll need and adapter to attach this N-connector

load to your BNC-connector transceiver



http://tinyurl.com/mxjgusn



Hard to beat for price but it does take a bit of

time to ship them over the big pond.



   Bob . . .

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Jul 23, 2014 7:14 am Post subject: Transmitting with no antenna?

At 08:07 PM 7/21/2014, you wrote:




Alternatively, you could build this dummy load: http://bit.ly/1wSMtI1



..using 20 of these (buy 25 for the price break): http://bit.ly/1lnVgfM



Note: Do *not* substitute wire-wound resistors!



Eric



   This is a time-honored and relatively easy methodology

   for crafting a low SWR dummy load. Built one of these

   when I was in high-school . . . my can was salvaged

   from some painting project around the house and I used

   Havoline motor oil . . . but as you can see, the time

   to fabricate this thing is not trivial. Be sure to store

   this thing rihgt-side-up . . . if it lays over then atmospheric

   breathing will push oil past connectors that are not

   liquid tight.  The commercial-off-the shelf alternative

   may prove more attractive.

   Bob . . .

edpav8r(at)yahoo.com
Guest

Posted: Wed Jul 23, 2014 8:38 am Post subject: Transmitting with no antenna?

On Jul 23, 2014, at 10:13 AM, "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com> wrote:

 	  Quote: 	
		   This is a time-honored and relatively easy methodology for crafting a low SWR dummy load. [SNIP] The commercial-off-the shelf alternative may prove more attractive.


	


Indeed.  I always seem to forget that you can buy virtually anything cheaply on eBay!  For $19, the economics of building one yourself, in $ and time, make little sense.



Eric



do not archive

donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Wed Jul 23, 2014 11:54 am Post subject: Re: Transmitting with no antenna?

 	  nuckolls.bob(at)aeroelect wrote: 	
		  

I was skeptical and disappointed when your conversation

with VAL suggested that 'your were probably going to be

okay' with the 10W resistors.



It would have been nice if the design I posted was

a little beefier . . . but getting resistors

with low inductance at VHF frequencies isn't a trivial

task. The venerable carbon composition resistor popular

to this legacy DIY task are not easy to find and not cheap

when you do find them. The 2W design proved to be the

best I could do with limited resources . . . although

suited to the task as long as you don't get into long-winded

test transmissions. As I recall, I produced that suggestion

as a test tool for proving that some appliance sensitivity

to transmitters was a function of energy radiated from

the antenna.  Short transmissions would suffice. 	




Uh... Val didn't say I would be fine with 10W resistors. 



They said that the radios should be fine. 



 	  Quote: 	
		  In regards to circuit protection, the COM 2KR doesn't have any active protection against keying the unit without a proper load attached.  As such, you run a small risk of damaging your radios keying them up without an antenna.  However, the radios are resistant to damage from being keyed without an antenna and should be fine.  	




I'm going to redo with the 1W metal oxide resistors. I'm only tracking down an audio panel switching issue. As such, I'm only keying the mic long enough to see if I get a transmit indicator on the display. So about a half second whenever I hit the PTT.

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Jul 23, 2014 12:00 pm Post subject: Transmitting with no antenna?

 	  Quote: 	
		  Indeed.  I always seem to forget that you can buy virtually anything 

cheaply on eBay!  For $19, the economics of building one yourself, 

in $ and time, make little sense.


	


    But we won't talk about the things we do, buy,

    read or just dream about . . . 'cause it's FUN!

    It's probably cheaper than a gambling habit . . .

   Bob . . .

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