NOOB to base setups... simple questions..

[DJEEPER]

OK! i have been thinking about getting a base setup... thats the first step into trouble, right?


little background: Im a college student, studying engineering, living in the room over the garage at my parents house and i would like to take advantage of the wide open roof.
There are trees around me...so i dont know how that will affect anything. My house is a 2 story with attic, so it sits a little higher than the average 2 story house.
I do not want to spend over 300 on this if possible...considering im a broke college student and all.

Here we go:

Can i buy a cobra 29 ltd classic, swing it up to 20 watts or so with a DK of about 4 watts, use a 102" whip for my antenna and run coax out my bedroom window to the roof?

i realize 110v outlet and 12v CB = fire, blue smoke of doom, death and destruction. So im looking at a power supply for my CB radio. I am also dropping the big dime on a Fatboy 900 Watt amp for my mobile, running the same type of radio and specs. The fatboy will be replacing my Palomar 250 HD Elite amp... which i would prefer to run inside my house.


ok... lets discuss :D

What am i looking at here?

-Cobra 29 LTD Classic
-RG58 COAX
-Power Supply (how many amps??)
-102" Antenna
-some sort of mount.


About the mount. im thinking about attaching a 4 legged horizontal mount on the edge of my roof... the antenna will mount on a horizontal pivot so i can flip the antenna 180* (down) during storms and such, then another 180* (up) to broadcast.

[DJEEPER]

this is a very very.... VERY rough draft.

pretty much allows the antenns to be flipped 180* after being unlocked from the upright position, in order to protect against lightning and such...



the grey triangle is some fixed counter ballast so allowe easy rotation of the whip, instead of fighting it up in the air...

[Foxhunter]

Well from the sheet I have in front of me this formula is best used for power supply computation: I= Px15%. So we are talking about your 900 Watt Fatboy....

900W x 15% = 135 Amps

It's always good to go a little over for breathing room, but that should be about your supply / demand.

Now you're using "RG58". There are different kinds of RG-58 and RG-59. TMS RG-58C is only good
for 300 v rms, while Belden RG-58C (8262) is good for 1400 v rms. Plain
old RG-58 (solid center conductor) is also good for 1400 v rms.

Now let us consider RF. 30 mhz is the worst case for HF amp use.
All ratings below are assuming 1:1 swr (VERY IMPORTANT) and 40 deg. C ambient.
RG-58 350 watts
RG-213 1500 watts
RG-8X 300 watts
RG-217 3900 watts

Whatever you use make sure it'll safely handle 1000W and the shorter the run the better for attenuation/loss.

The idea of a fold-down antenna is a good one and many Amateurs do it, for good reason. Some have towers on which the mast is hinged and is lowered down in anticipation of bad weather or for maintenace or servicing. Let us know how you're making out, I saw your post go unanswered so I thought I'd atleast start. Some guys have trouble with having their radio room upstairs as their ground wire is often long (too long) and it can lead to problems.

[DJEEPER]

oops... i think i was misleading. the 900 watter is heading into my jeep.

the palomar 250 HD Elite is heading into my house....

hope that clears it up some!


(on occasion, i might swap the two just to blast out to japan or something )

[Foxhunter]

Check the max wattage output on the amp and/or what transistors are in there. If it's a 250 Watt amp------250W x 15% = 37.5 Amps needed with 8 AWG supply cable. Always allow a little more than minimum that's required. The RG58 should be OK. Like I said, being upstairs with a long ground cable can cause problems sometimes as it can become part of the radiator/RF hot so keep that as short as possible. Let everyone know what and how you're doing......

[Circuit Breaker]

The 102" whip won't work as an antenna unless it has three 102" radials attached to the ground side of the coax to give it a ground plane.

[Circuit Breaker]

(on occasion, i might swap the two just to blast out to japan or something )

That won't happen unless the band is open. You could push 10,000 watts and never make it to Japan unless conditions were right.

[KI4MSJ]

First off. You will need a better antenna. Either vertical or Dipole. The 102" just won't work well. As far as the power supply IMO bigger is always better. I would go wit ha 50Amp variable voltage like an Astron. This will run your 29 and 250 amp with more than enough left over. Get yourself an Antron 99 for real cheap and use wall mounted mast mounts for a 10' mast, run a groung strap to a grounding rod straight down from the mast.

[Circuit Breaker]

I wasn't completely correct on my first post. A 102" whip won't work well as antenna unless it has some kind of groundplane...that part is true. However, if the antenna is more than a 1/4 wavelength above ground, as it appears it will be, you can get away with just two radials connected to the "ground" side of the antenna mount. They don't have to be 102" whips....it can be just an equal length of wire.

[drdx]

I don't know where that 15% formula came from but I've NEVER seen it. If you go by that you'll have a lot of head room for sure though, as it is wrong way to the high side, like by 50% in that case. You can either use the EIR basic electronic formula, or just figure in 10 amps per 100 watts and be fine. So, for 250w, you're talking about 25 amps. You can do the old Wattage/divided by volts and get the amperage but nothing is 100% efficient. That would be a 250 divided by 12 volts for the 20 amp range, but that leaves out efficiency.

That antenna will work fine with some radials as stated, but with power and an antenna that low you're asking for trouble in the form of TV and consumer electronics interference. Unless you live on a farm well away from neighbors, you'll have issues. If you have that whip, and 3 radials sloping downward that are about 8.5 foot long, you basically have what used to be manufactured as a Starduster M400, just a little different in materials, and they were a TV killer. When you aim the radials down you they radiate and there is said to be a .3 db gain picked up there but it will invite the interference. That's why many antennas of that era ran the straight horizontal radial systems to let it act more like a ground plane than a vertical dipole with 3 shield fed legs.

-drdx

[Circuit Breaker]

I think the reason the radials on the Star Duster antennas were sloped downward is because it lowered the impedance closer to 50 ohms., and, as you kind of eluded to, it lowered the angle of radiation.

[drdx]

Yep, it does, and thanks for pointing that out. I ran a starduster for my first base antenna back in '83. I just searched the web and you can still get them. I think antenna specialists made them back then. -drdx

[Foxhunter]

I don't know where that 15% formula came from but I've NEVER seen it. If you go by that you'll have a lot of head room for sure ..... -drdx

I have an "Amp-Guide Data Sheet" from a shop-owner that I know in the tri-state area here an was given the formula (I=Px15%) An example on the sheet:

250 Watts x 15% = 37.5 Amps = 8 AWG wire/cable (these are for runs 12 Ft or less)
500 Watts x 15% = 75.0 Amps = 2 AWG wire/cable

The "technician" who gave the sheets to me is a Radio Technician With An Electrical Engineering
Degree From Penn State University and holds a 1st Class FCC Radiotelephone License.
He worked At General Electric For 11 Years Before Opening His Amateur Radio & CB repair center.
His son graduated From Pennsylvania Institute Of Technology With An Associates Degree
In Electrical Engineering, Electronics Technology, & Computer Technology. He Also Holds A 2nd Class FCC Radiotelephone License. They do factory authorized repairs and warranty work for Ranger Communications Inc and are used to test new prototype models produced by Ranger. I have alot of faith in the information I'm given and they provide. I'd agree that the ratings are a little more than minimum or adequate but as a safety the supply will be certainly enough. I'm surely not trying to get the guy to throw away his money with "overkill" and am sorry if I've misled him but it would be wise to leave a little additional "headroom" when it comes to supply. No harm no foul.

[DJEEPER]

ok, so i have some more researching to do.

like i said, im pretty new to the concept and now realize its a WHOLE bunch more work to get a base station started than a mobile....

Thanks for your inputs and i will be getting back to this once i do some more reading on here....


Good input, guys. :D