Avoiding transmitting

[electronicskip]

I always without fail use a dummy load when testing any type of transmitting equipment , more so in case I key the microphone by accident and blow the unit .
So its always the first thing I do before doing anything internally.

[David Simpson]

I use a Marconi TF1020/A RF Power Meter as a dummy load. For studying the modulated o/p's envelope - a short length of 50 ohm co-ax cable's inner core applied close to the Tx's o/p stage - but not touching anything ! - can be plugged into a suitable 'scope's BNC socket.
I ken nothing about a WS88, but I bet there are hundreds of knowledgeable folk on the VMARS Forum site. However, what everyone has said about correct antenna circuitry loading must be observed with all Tx o/p's in general.

Regards, David

[HamishBoxer]

I think in reality a 47Ohm res (2W) would suffice.

[AC/HL]

It's a long time since I worked on PMR, but I used to use a load with a sniffer output (or input). Saves the risk of accidentally damaging your signal generator.

[Tyso_Bl]

I'm not to sure if this is a legal or technical question or a bit of each, from the technical perspective a transmitter needs to be connected to a load, if you don't want to transmit that means a dummy load as advised by previous posters, as the eqipment isn't in Band 2 or the illegal CB bands there isn't as I understand a prohibition on posession.

The question of transmitting then becomes a legal one as to your intent. If you've connected an aerial then your intent is to transmit, if you've connected a dummy load there is no intent to transmit, difficult to see how you'd get in to trouble even with a little leakage.

I've had an official visit as a result of soak testing a 1.4kW setup, on various amateur and non amateur frequencies, best practice followed re screening, and they had to go away.

Don't be too paranoid, if it isn't specifically prohibited, it's allowed!

T

[Boom]

If you use two identical helically cut carbon film resistors and connect them with their ends reversed it should cancel out any inductance.

[Herald1360]

Not sure about that. Won't it depend on there being tight coupling (high mutual inductance) between the resistors, which is unlikely in practice.

[Graham G3ZVT]

Originally Posted by Tyso_Bl

I'm not to sure if this is a legal or technical question or a bit of each, from the technical perspective a transmitter needs to be connected to a load, if you don't want to transmit that means a dummy load as advised by previous posters, as the eqipment isn't in Band 2 or the illegal CB bands there isn't as I understand a prohibition on posession.

The question of transmitting then becomes a legal one as to your intent. If you've connected an aerial then your intent is to transmit, if you've connected a dummy load there is no intent to transmit, difficult to see how you'd get in to trouble even with a little leakage.

I've had an official visit as a result of soak testing a 1.4kW setup, on various amateur and non amateur frequencies, best practice followed re screening, and they had to go away.

Don't be too paranoid, if it isn't specifically prohibited, it's allowed!

T

Lets explore those points by way of this illustration.

A video sender operating in Band IV/V like this:

http://rf-links.com/newsite/transmit...nder-ntsc.html

is specifically prohibited, including mere possession, "Having in one’s custody or control".

(The Wireless Telegraphy (Control of Interference from Videosenders) Order 1998)
http://www.legislation.gov.uk/uksi/1998/722/made

So it appears your "intent" (other than to re-export it) has nothing to do with it.

However, anyone can buy one of these:

https://www.toolstation.com/shop/Ele...dulator/p80695

And connect it to something like this:

https://www.toolstation.com/shop/Ele...dulator/p80695

Now we have something that is probably more powerful than the "banned" video sender, and as long as we use it for its intended purpose of TV distribution in co-ax cables and don't connect an aerial to the output, we are within the law.

But what if we wanted to use the "banned" device in the same way, in a TV distribution head end? Well it appears we can't.

So what exactly puts a device in the category banned by the above mentioned legislation?

The label on the front of the box?

[TonyDuell]

A few more thoughts :

1) I have no intention to transmit with this unit. I think I said that at the start. And therefore I have no intention to connect an aerial

2) A dummy load is a good idea (even if not strictly needed), so I will use one. The only issue would be screening it. The WS88 does not have a coaxial aerial connector. It has a simple socket into which, I guess the whip aerial fitted (I do have a plug to fit it). Near it on the panel, 1-2 inches away, is a similar (smaller) earth socket. I can connect a resistor between them, but it could still radiate. Should I make up a metal box with 2 pins (One to the box, one insulated from it) to plug in there with the resistor inside?

[Sean Williams]

To be honest Tony, I think you are rather over thinking this - the WS88 running into some form of resistive load isn't going to radiate a significant quantity of RF - certainly if you are going to run the transmitter up out of it's case, there will be more RF leakage from the internal components and wiring than there will be from your load.

In all of these things, best efforts are the key here - if you were running a professional grade repair workshop, you would be expected to have all sorts of measures in place to stop radiation of unlicensed signals, but you are a hobbyist, so in your case, running into a load would be appropriate for the level of work you are doing.

If you were planning weeks and weeks of continuous operation of the transmitter, it would be prudent to take greater measures, I would suspect you are running the transmitter up out of curiosity, and then only to take measurements - in this case, I would just get on with it, taking sensible precautions to limit any radiation to an absolute minimum.

[CambridgeWorks]

I agree with Sean. I doubt with your intended connections you would radiate as far as the pantry transmitters do.
My job used to be at a manufacturers as a customer service engineer. The product was air band fixed station transmitters and receivers. Max power 265W of vhf am.
The occasional accidental transmission at 121.5Mhz or 243Mhz (40W am) distress frequencies did happen whilst testing, but always into a proper rated good quality rf load.
I was told that our location was aware to the authorities in case a signal was ever traced to it. Whether that is true, I do not know.
However, radiation from a host of occasionally keyed transmitters in our hot room test chamber never caused problems with other receivers in the immediate vicinity.
The extremely low level of radiation you might occasionally omit should not worry you.
Indeed, 20 or so years ago I had a very old General Radio "Standard" sig gen, 50khz to 50Mhz IIRC. You could certainly radiate a decent signal with it with a glowing torch bulb on the output!
By seeking advice here, you are seeking to minimise any problem. That shows a very responsible approach IF EVER you were taken to task. I would imagine that nobody has ever been prosecuted for using an 88 set into a home made dummy load whilst out of it's case. Back in the 60s maybe when deliberately transmitting into an aerial, yes, possibly. Back then, the authorities (GPO) had the staff to enforce obedience and could at times be over zealous in their actions. Today they don't have the money or the staff!
Rob

[G8HQP Dave]

Originally Posted by Boom

If you use two identical helically cut carbon film resistors and connect them with their ends reversed it should cancel out any inductance.

I note that this was posted on 1 April.

[tealandsilver]

Use with a dummy load as others suggest. The WS88 is a vhf fm low power set anyway, I think. Operation with the metal covers removed could lead to a bit of localised sporadic rfi, but will probably be drowned out by all the other emi sources around. Provided it doesn't interfere with the Missus's Corrie viewing you should be ok!

[TonyDuell]

Originally Posted by Tyso_Bl

I've had an official visit as a result of soak testing a 1.4kW setup, on various amateur and non amateur frequencies, best practice followed re screening, and they had to go away.

I feel there is a big difference (in terms of interference produced) between a 1.4kW transmitter and a 0.25W one (which is what I believe the WS88 is). I also feel there is a big (at least moral) difference between running a transmitter into a dummy load for less than a minute and seeing if anything is getting out and a 'soak test' which I would think went on for some considerable time.

[emeritus]

In the 1970's I was doing the system test on a prototype solid state transmitter, circa 20W in the 200 - 400 MHz band working into a dummy load. Signal leakage was traced to the use of BNC connectors. Replacing them with type N and SMA screwed connectors completely cured it, and we could sit the receiver on the transmitter casing and wind up the attenuation and completely lose the signal with no trace of breakthrough, even though we were using ordinary 50 Ohm cable and not the special double screened stuff. The customer we were designing it for had expected that they would have needed to build a special screened room for their own testing.

[Boom]

Originally Posted by G8HQP Dave

I note that this was posted on 1 April.

http://www.resistorguide.com/inductance/

[G8HQP Dave]

I can't see anything there about reducing inductance by connecting two together. As Herald1360 said, you would need high mutual inductance (and of the right sign) - and you won't get enough just by putting the two resistors next to each other.

[G6Tanuki]

I truly can't help but feel there's spectacularly-unwarrantedly-deep levels of analysis going on here.

Sure, the WS88 is crystalled-up on "military" channels - but:

[1] the MoD have not been using old Larkspur-era or earlier radios on that band for decades. They've been all-digital on Bowman since the early-2000s.

[2] If a few hundred Milliwatts of RF radiated on legacy frequencies poses a significant threat to national security then I wonder what we're spending billions of quid per year on our defences for.

[From memory of work with various Larkspur tactical "C"-numbered radios in the 1970s, when there was a 'lift' on and European/Soviet-bloc/Middle-Eastern TV stations were being received at S9++ everywhere between 30 and 80MHz we either just tuned-round to find a frequency that currently wasn't blocked, or we found a friendly Air Force type whose radios worked in the 225-400MHz band].

Don't be paranoid: your milliwats of leakage RF are not going to result in a 3AM visit by the Stasi.

[Herald1360]

Originally Posted by Boom

http://www.resistorguide.com/inductance/

Aren't April Fools pranks after midday invalid anyway?