Ham radio questions

@ Her name is lucy.

This is a good question. Please understand that this knowledge is something that all LICENSED hams study and need to know.
It seems, no matter how many times a myth is dispelled, they keep raising their ugly heads. One common myth is that power cable fuses protect the radio from damage. They don't. Myth number two is, that a fuse will open instantly as soon as the current exceeds the fuse's rating. It won't. Myth number three is, it is always okay to use fuses designed for 120 volts AC, in a nominal 12 volt DC system. It isn't.

Fuses are there to protect the cabling. For example, the Icom IC-7000 has a 5 amp (system) fuse mounted inside the radio, and 30 amp fuses in the cabling (plus and minus). If you short out a supply connection (pin 3 of the tuner port for example), a circuit board trace and/or switching transistor will fail long before the 5 amp fuse opens. The 30 amp fuses will never open in this particular case. It can be argued that the power cable fuses do protect the radio if something fails catastrophically, a final perhaps, but chances are some other component in the circuitry will be damaged beyond repair before the power cable fuse(s) opens.

The definition of a fuse is relatively simple. It is a wire that melts when subjected to too high of a current. When it does, the circuit opens... hopefully. I say hopefully, because if you've chosen the incorrect size for your application, it may not open. Or, it may open after a long delay. In any case, you want the fuse to do its job, well before your wiring becomes its own fuse!

Unless you're an engineer, you don't hear the word hysteresis very often. Hysteresis is used to describe a phenomenon, in which the value of a physical attribute lags behind changes in the effect causing it. I use it here because fuses are a prime example of the hysteresis effect. This fact figures heavily on our choice of fuse amperage ratings, the wire size we choose as a result, and ultimately how safe our installation is should a dead short occur.

All fuses exhibit hysteresis. This is the time lag between any given ampere overload and when the fuse opens (I2T). For example, a nominal 20 amp fuse will handle a 30 amp load for about 90 seconds. It will hold a 100 amp load for about 1 second. This is one reason slow blow fuses are not recommended for amateur devices, as their I2T is considerably longer.



Note that a 30 amp Maxi fuse will take about 3 seconds to open when subjected to a 100 amp load! The same fuse will carry 40 amps for about 2 minutes! As the static temperature goes up, the vertical scale compresses slightly, and in very cold temperatures settings it elongates slightly. So, hysteresis is the time lag between applying an overload, and the fuse opening to protect the wire. Here is a pdf with more data on the Maxi fuse.

In the mean time, the wire being protected is getting rather warm. If it gets too warm, hot really, it could cause a FIRE!!!!!

 

mrrobwood, that is an excellent answer. I never realized a fuse would withstand a higher current draw than it's rating for a brief period of time before it opened, at least not as long as you said it would. Thought that's why they invented "slo-blo" fuses for that characteristic.

I was looking at the cigarette lighter fuse protection the truck wiring, not so much protecting the radio. Figured the radio would have its own internal protection, which would neither know or care where the current it sees is coming from.

I don't understand though, if the truck wiring is supplying sufficient current for the radio, what is the difference between drawing the rig's power from there as opposed to straight off the battery? It seems even if the truck's lighter wiring fuse is useless for protecting the rig, it would still be some protection as opposed to zero protection when hooked straight to a battery capable of supplying 300+ amps for a brief period of time.

I asked an electrician friend what the difference between using a 120v rated fuse and a 12v rated fuse is...he replied "Ohm's law doesn't apply here, the fuses will still blow at the same amperage rating, the only difference is a 120v rated fuse will open with sufficient distance in the gap to keep 120v from arching across the blown open element, whereas a 12v rated fuse doesn't have to worry about that so much". In hindsight I should have asked about circuit breakers (just for curiosity's sake).

Interesting to see the difference between an electrician's and a tech's view. One brute force, one finesse.

 

I wired it directly to the battery about 2 months ago and have had zero problems transmitting 75 watts on 2 meters. That was from advice given to me on a ham radio forum specifically for mobile operating.

Ham is very useful. I am in Colorado right now and have been driving on forest service roads out in the mountains where cell reception is nonexistent; even though I'm 30 miles from the nearest repeater I can still reach it from my truck with a shitty $20 magnet antenna with 5 watts power.

When I get home I am planning on posting a bunch of pictures but not now as I only can get internet at Starbucks and have to take care of more important business first.

Thanks for all the replies folks.

 

Bumpity! Love the Ham info! Also, please post pictures of your installs.

 

I just got an Icom 208H. It is mounted under passenger seat and has a detachable face that is in the sunglass holder. Very clean looking!

 

If I were on same frequency as the repeater at the ski hill I am at can I legally use a HAM radio like the normal radios we use at the hill or would I still need to say my call sign and such? I haven't got a radio or license yet, I am still looking into it.

 

Y'all have intrigued me about ham...

 

I'm Looking at putting together a setup using a Yaesu FT-2900R. I would appreciate any feedback from anyone that runs this radio. I would also enjoy seeing photos of everyone's install to get some mounting ideas. Thanks!

 

i have the older version FT-2800m and i love it. they are great radios and never had a single problem with mine. i also have the same radio in my rockcrawler as well. i recommend that you get the programming software because these radios are ridiculous to program by hand if your are programming repeater pairs with CTCSS tones. soon tho , will be upgrading to a 440/144 radio but haven't decided which model yet.



i also have a NMO mount antenna drilled into the roof

 

thats somewhat the setup I want to put in my truck

 

That is a sweet set up. Thanks for posting pics! Are you using an external speaker or is the internal loud enough when mounted in the dash?

 

You guys are way off on a cig plug not being able to run a 50W radio.

A 50W radio will generally draw 10 amps which is easily within the capability of any vehicles cig plug that I've ever seen. The voltage drop is minimal.

I install these for a living (in addition to being a ham) and none of the customers that have requested the cig plug install have had a single problem, some in 5-6 years of steady operation.

Wiring to the battery is totally not necessary. It also will not work if you want your radio to come on with the key and it does not have a sense line.

Not to say to the battery directly won't work, but it is far from needed.

 

my radio (Yaesu FT-2800) is 65watts, the Yaesu FT-2900 is 75watts of pure power. the Icom 208H is 55watts of awesomeness.

 

Ham is hardwired to battery, CB is on cig plug.

 

75 watts is still well within the power supply ability of a 20 amp cig plug.

 

I wired it to the battery because I need the other cigarette lighter plug for my iPod charger.

 

Wiring to the battery is just fine, I just wanted to counter the misinformation that plugging into the cig plug will wreck something and should be avoided at all costs.

It isn't the cleanest way of doing things but there is nothing wrong with it.

 

Here's a pic of my setup

 

If the repeater is using a frequency set aside by the FCC for HAM (most likely it is) than you would need a license with privileges to use that frequency.

Getting your license is easy and cheap. Go ahead and get it.

 

The repeater is licensed for the operations of the ski hill. I have permission from the ski hill to be on that channel. I am ski patrol, as soon as I get there I have a radio in my hands. So I guess my question is am I allowed on that frequency, and legally able to use it like I would my normal issued radio. Or do I have to follow all amateur radio laws. I plan on getting my License at somepoint soon. Right now I have enough studying for the ski patrol.