Can using a lower voltage power supply ruin an FM transmitter?

[ke5frf]

Well, I do realize that having the AF too low creates a poor quality signal. Perhaps you'll find a good deal on a more suitable attenuator. You might check out a site called MFJ that specializes in amateur radio equipment.

 

[Alcyone]

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[Alcyone]

Do you think a 2w attenuator would work such as these?

Attenuators - Tektronix Attenuators - Cal Test Attenuators - Pomona Attenuators

 

[ke5frf]

Do you think a 2w attenuator would work such as these?

Attenuators - Tektronix Attenuators - Cal Test Attenuators - Pomona Attenuators

If I were you, I would call the manufacturer of the attenuator and discuss in detail what you are doing. It is difficult to know precisely how much power your radio is going to be transmitting at half voltage. It is going to be borderline at two watts. The manufacturer can at least advise on what level it can handle as far as headroom goes. My personal thought is you'll be ok with a 2 watt job if you can add some kind of thermal transfer technique like a heatsink...perhaps a heatsink and a computer fan. I don't know what all of this is for and your financial investment but I'd go with a higher rated one myself.

 

[Alcyone]

I was able to get it down to 6.5 volts. Would you say that would make it safer to to use the 2 watt attenuator or do you still recommend a fan and/or heat sink?

 

[Damo666]

If you must transmit on band 2 vhf, you should've bought something professional from, say here FM Transmitter Kits and Radio Transmitters - Aareff Systems . That cheap Chinese Hlly junk would be better in the bin. Wont be long before you're " knee deep " in dung if you carry on Txing with that thing. The harmonics transmitted from those will cause mayhem to every man and his dog. Just don't say you've not been warned. Good luck!

 

[ke5frf]

I was able to get it down to 6.5 volts. Would you say that would make it safer to to use the 2 watt attenuator or do you still recommend a fan and/or heat sink?

If it were me I would be rigging up a heatsink anyway. But, I think you are on the edge of being ok. Just don't say you weren't cautioned.

 

[Alcyone]

If you must transmit on band 2 vhf, you should've bought something professional from, say here FM Transmitter Kits and Radio Transmitters - Aareff Systems . That cheap Chinese Hlly junk would be better in the bin. Wont be long before you're " knee deep " in dung if you carry on Txing with that thing. The harmonics transmitted from those will cause mayhem to every man and his dog. Just don't say you've not been warned. Good luck!

I'm not a fan of cheap chinese products but I can tell you it doesn't interfere with satellite radio, FM (except the frequency broadcasted on), AM, SW1, SW2, SW3, TV (UHF or VHF) or my garage door opener. Nor does it interfere with my wireless router, wireless AV system, CB radio or Cell phone. That being the case, not every man and his dog. Maybe just the dog if at all.

Do you by chance work for "Aareff Systems"?

 

[Damo666]

No, i don't work for Aareff. I just thought their gear looked good quality without costing a fortune, hence the link. Though you say your Transmitter isn't producing spurious emissions - have you connected it to a spectrum analyser, or even tried the cheap and cheerful way of using a scanner? I don't want to get into flame wars on this, i'm simply suggesting be careful.

 

[mbarazeen]

if you have a RF watt meter you can easily know your power out put, and connect the available attenuator.
if you dnt want to use an antenna and what you need is to power your transmitter and get signals just in your house only. you directly make a load for your transmitter by using parellel 1W, 250 ohms resistors(5 nos) to get 50 ohms load. then the power will be completely discipated on the load since it would be purely resistive. some leakage of RF signal would give you good signal strength in you house.

 

[Alcyone]

No, i don't work for Aareff. I just thought their gear looked good quality without costing a fortune, hence the link. Though you say your Transmitter isn't producing spurious emissions - have you connected it to a spectrum analyser, or even tried the cheap and cheerful way of using a scanner? I don't want to get into flame wars on this, i'm simply suggesting be careful.

I understand and appreciate your concern ans spurious emmissions, harmonics are a good reason for the FCC to come after you. Are you referring to a police scanner? I tried a uniden 16 band scanning radio and that didn't pick up anything when the transmitter was on. Is that what you're referring to?

 

[Alcyone]

I am looking at this manual which I think i'll give a shot. I have plenty of heat sinks laying around from old computers, etc.

https://www.electro-tech-online.com/custompdfs/2009/12/dl_30w_hf-uhf-1.pdf

I see how you connect the resister to the BNC connector. I assume the transmitter then connects to the BNC connector? So where does the antenna come in? It seems like a one ended doummy load. Does the signal actually transmit from the dummy load without an antenna? Am I missing something?

Thanks

 

[ke5frf]

I am looking at this manual which I think i'll give a shot. I have plenty of heat sinks laying around from old computers, etc.

https://www.electro-tech-online.com/custompdfs/2009/12/dl_30w_hf-uhf-2.pdf

I see how you connect the resister to the BNC connector. I assume the transmitter then connects to the BNC connector? So where does the antenna come in? It seems like a one ended doummy load. Does the signal actually transmit from the dummy load without an antenna? Am I missing something?

Thanks

I believe that is the "homebrew" dummy load that I suggested you look at earlier.

All it is, frankly, is a load resistor. But to understand why it does transmit "leakage" electromagnetic energy you have to investigate and learn about AC circuits, particularly RLC (resistive, inductive, capacitive) circuits and filters.

All an antenna amounts to is a resonant band-pass filter. Antennas by their nature are capacitive, inductive, and resistive elements. When an antenna is designed to be resonant at a particular frequency, it will present a 50 ohm IMPEDANCE, well actually 68 ohms but that is an unimportant theory tidbit, which is a combination of inductive and capacitive reactance as well as resistance, to the amplifier stage of the transmitter. This is too complicated to describe in a few paragraphs. Just know that your supplied FM transmitter antenna is designed this way.

When the antenna is resonant the capacitive and inductive properties will be in balance, leaving the characteristic impedance of the feedline to essentially be the only thing the transmitter sees, which in this case is 50 ohms and purely resistive. The feedline has its own inductive, capacitive, and reactive elements as well. You can actually cut a piece of coax to a certain length and the transmitter will see 50 ohms without an antenna attached. A 50 ohm resistor at the end of a short piece of coax will take the place of the 50 ohm resonant impedance of the antenna and present the proper impedance to the transmitter. However, a resistor is not a reactive element and thus will not, in a perfect world, radiate an electromagnetic field, which is the property of inductors and capacitors.

HOWEVER, a resistor is never a perfectly resistive, non-reactive component. The leads of a resistor are straight wires and will radiate as an inductor. The leads are often within proximity to each other and thus form capacitive plates. In some resistors there is a shunt impedance in the resistive element itself and small series capacitance within. The higher the frequency, the more these reactive conditions within the resistor become evident.

It is best to choose a resistor for a dummy load that mitigates much of the reactance that other resistors can't. For one, because you can't predict that the impedance will remain 50 ohms without doing calculations. The Thin film resistor in the project is a good example of one that will have minimal reactive properties. But no resistor is perfect in the real world so a 100 MHz even this resistor will radiate a certain amount.

This is why dummy loads still manage to radiate. Between the connectors, the leads, and the internal reactance in the resistor, you will have leakage radiation. In the case of 5 watts terminated into it, you will probably have plenty of radiation to be picked up by a FM reciever within the confines of your property, on an unused frequency where no interference can occur.

This is why I suggested building a dummy load from the start.

I hope the discussion about AC theory wasn't too confusing. I may have missed a few things in trying to simplify the concept but you can learn more on the internet.

 

[Alcyone]

I understand. Thanks for that explanation. However, I'd really like to keep use of my antenna since it's in the attic and would more evenly distribute the signal throughout the house.

Since 5 watt BNC connectors are hard to find above 1 watt, could I use something like this:

**broken link removed**

Then, use a N to BNC adapters?

What would 6 db get me as far as power?

Thanks

 

[ke5frf]

That attenuator looks like it is strictly designed for upper end UHF frequencies. This is a good example where very high frequencies react differently than lower frequencies...i.e. the capacitive and inductive properties have to be tightly calculated to permit components to work properly. On 100 MHz it is likely the impedance of this attenuator could be considerably off from 50 ohms and thus damage your transmitter.

I do understand your desire to use the actual antenna. But unless you live at the Playboy Mansion or Biltmore Estates or some other terrifically large mansion you won't notice the difference between the antenna and a simple load resistor.

I'd prefer the antenna too, and the attenuator. But not for that particular reason.

 

[Alcyone]

I decided to make a dummy load with a heat sink as shown in the pdf diagram you posted. I was wondering if it's sufficient to ground the resister to the heat sink or does it need to be grounded to the BNC connector?

Thanks

 

[Alcyone]

I have made a dummy load, but I had to attach a piece of copper wire to the resistor ground wire because it wasn't long enough. I used a wire nut to connect them. Is it OK to do that? I wanted to make sure before I hooked this up to my transmitter.

I have tested for continuity, but the strage thing is I'm getting a little bit of continuity between the ground and the center tube inside the BNC connector. Is this normal?

 

[killivolt]

I would not ground to the heat sink it appears to be a I/O in out to the BNC.


When you connect anything that will maintain a constant wattage I recommend you make your surface connection either soldered or as you can see it is both flat surface connection and soldering of the resistor to BNC.


kv

 

[Alcyone]

The resistor is soldered to the BNC. The othrer end of the resistor is wire nutted to a piece of copper wire, the other end of that wire is then grounded underneath one of the screws on the BNC connecter.

This should work as is, but I'm not an electronics expert so I just wanted to make sure I didn't do something stupid.

 

[killivolt]

The resistor is soldered to the BNC. The othrer end of the resistor is wire nutted to a piece of copper wire, the other end of that wire is then grounded underneath one of the screws on the BNC connecter.

This should work as is, but I'm not an electronics expert so I just wanted to make sure I didn't do something stupid.

Your right. Sorry about that I just noticed in the drawing that it was grounded to the heat sink. I still would make all my connections soldered. If you want to wire nut it afterward to guard against accidental shorting otherwise just heat shrink it.

kv