Attenuator for too powerful FM transmitter.

[JeffF]

Just added a 1W, 10dB attenuator to my 1W Chinese brand FM transmitter and I have to say I'm now happy with the reduced range.

Before adding the attenuator I was able to get my signal in every direction at least 2 miles(and up to 3 miles on the downhill side of our house). Last year this concerned me enough to get a brick wall filter for everything above 108MHz. Big reason is I'm less than 2 miles from our local airport and frequencies above 108MHz are theirs. I don't know how dirty my transmitter is but I didn't want to chance *anything* in regards to the FAA.

Even so, I didn't like that I transmitted soo far. I would have replaced the transmitter but it has excellent sound. Figured a 10db attenuator would get me to ~0.1w. And on top of that, I dropped the supply voltage to the transmitter from 12.5v to 9v so that's even less power going out.

I now have crystal clear FM to only about 600ft... which is exactly what I want. You pick up the radio when you see the lights and not much further.

Here's the attenuator I bought...
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=220500049730

The guy took my $7.00 best offer so it wasn't too bad at all.

Just putting this out there in case there's someone else in a similar situation.

-Jeff

[Denis Chaput]

What type of transmitter did you buy??

[greenie95125]

Jeffrey wrote:

Just added a 1W, 10dB attenuator to my 1W Chinese brand FM transmitter and I have to say I'm now happy with the reduced range.

Before adding the attenuator I was able to get my signal in every direction at least 2 miles(and up to 3 miles on the downhill side of our house). Last year this concerned me enough to get a brick wall filter for everything above 108MHz. Big reason is I'm less than 2 miles from our local airport and frequencies above 108MHz are theirs. I don't know how dirty my transmitter is but I didn't want to chance *anything* in regards to the FAA.

Even so, I didn't like that I transmitted soo far. I would have replaced the transmitter but it has excellent sound. Figured a 10db attenuator would get me to ~0.1w. And on top of that, I dropped the supply voltage to the transmitter from 12.5v to 9v so that's even less power going out.

I now have crystal clear FM to only about 600ft... which is exactly what I want. You pick up the radio when you see the lights and not much further.

Here's the attenuator I bought...
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=220500049730

The guy took my $7.00 best offer so it wasn't too bad at all.

Just putting this out there in case there's someone else in a similar situation.

-Jeff

Wow, 2 miles with 1 watt? What type of antenna? How high up do you have it mounted? I'm in a dense residential neighborhood, and can't get nearly that far with 20W.


Merry Christmas!

--Mike

[JeffF]

I bought it off eBay last year... it was a VastElect brand and it didn't come with a case... just two PCBs. I think I paid ~$50 delivered for it if I remember correctly.

And as far as antenna goes, I'm just using the adjustable ground plane antenna they included with the transmitter and it's up on the peak of the roof.

One thing I have going for me for distance is we're at the highest point in our county. And it's very rural so there isn't much around(except for the airport as mentioned).

-Jeff

[greenie95125]

Jeffrey wrote:

I bought it off eBay last year... it was a VastElect brand and it didn't come with a case... just two PCBs. I think I paid ~$50 delivered for it if I remember correctly.

And as far as antenna goes, I'm just using the adjustable ground plane antenna they included with the transmitter and it's up on the peak of the roof.

One thing I have going for me for distance is we're at the highest point in our county. And it's very rural so there isn't much around(except for the airport as mentioned).

-Jeff

OK, your location makes a HUGE difference, as does the placement of the antenna. Makes sense now.

I'd be concerned about the airport as well. Overmodulation can be a bigger enemy than over power. Make sure your broadcast is of lower volume than the commercial broadcasters around you, and you'll be ok.

I broadcast on 88.7, and there's a licensed low power station about 5 miles away on 88.5. They overmodulate, and bleed into my signal at about 1/2 mile from me. It's a low power trick to extend range a bit. I can't really complain now though can I?



Merry Christmas!



--Mike

[Alcyone]

Where did you get your 9v power supply? I have a 5w HLLY transmitter that goes to far. I'm having a hell of a time finding an adaptor tip for a universal. Radio shack doesn't have one that works. I can't find anywhere online to tell me the type or demensions of the adapter it came with.

[greenie95125]

Alcyone wrote:

Where did you get your 9v power supply? I have a 5w HLLY transmitter that goes to far. I'm having a hell of a time finding an adaptor tip for a universal. Radio shack doesn't have one that works. I can't find anywhere online to tell me the type or demensions of the adapter it came with.

Just splice it yourself. It's easy.

Get the 9v power supply you need, and cut the plug off (not the plug that goes into the wall socket :shock: ). Then cut the plug off the hlly power supply (leaving a foot or so of cord). Now, splice the 9v power supply to the hlly plug. Make use you verify which lead is + and - first. a cheap multimeter won't hurt if the power supplys aren't marked properly.

The ID looks something like this:

-minus ------------(.-------- + plus That means that the outer conducter on the plug is negative, and the inner conducter is positive.

BTW... how far are you getting out on that 5w hlly transmitter? Are you using the GP antenna they provide, or something different?


--Mike

[Alcyone]

Thanks for the reply. I was able to find the right tip. It's a type L with 5.0 mm ext diameter. 2.5 mm diameter. I'm running the transmitter on 7.5 volts. However, being that I'm using a 5 watt transmitter this hasn't cut my power output down to legal levels. Do you know where I could get a cheap attenuator that's capable of handling 5 watts?

[greenie95125]

Alcyone wrote:

Thanks for the reply. I was able to find the right tip. It's a type L with 5.0 mm ext diameter. 2.5 mm diameter. I'm running the transmitter on 7.5 volts. However, being that I'm using a 5 watt transmitter this hasn't cut my power output down to legal levels. Do you know where I could get a cheap attenuator that's capable of handling 5 watts?

Is there no way to reduce the output power of that rig? My PCS 20 watt rig can be adjusted from .5w to 20w from the front panel. On less expensive units, there's usually a pot on the PCB to adjust the output power.

--Mike

[Alcyone]

I don't think so. This is a very cheap unit. It' doesn't even have an on/off switch. The only controls are to adjust the frequency. However, I must say I'm impressed with the 3 mile radius I tested this thing at.

[greenie95125]

Alcyone wrote:

I don't think so. This is a very cheap unit. It' doesn't even have an on/off switch. The only controls are to adjust the frequency. However, I must say I'm impressed with the 3 mile radius I tested this thing at.

I'm impressed as well. Are you in a desert?

--Mike

[Alcyone]

No I'm actually in a suburban area with a few slight hills here and there. The antenna is in my attic. If I were to get a real FM antenna and mount it above my roof on a pole, I'm sure you could add a couple of miles to that range. And just to be clear, it's not a static free 3 mile radius. That's about how far I can hear it on a car radio.