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Author Topic: Mobile Amp  (Read 799 times)
N4GRN
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« on: November 09, 2002, 12:22:11 AM »

I am preparing to take my cw test within 4 weeks. I have passed the General and Extra written exams and am setting up my mobile rig for my next trip to my home in Nova Scotia. I hope to be on HF this next trip in the auto as I am also going into Newfoundland and Labradore and will be traveling along coast line all the way and camping. So I will have lots of time for radio.

I am looking at adding an amp to my mobile rig. I have looked at the SGC-500 and the Skywalker 500 or maybe the Skywalker 1000.

Does anyone have any comment on either of these amps. I can build the Skywalker 1000 for almost 1/2 what the SGC 500 cost and have double the power when needed.

I have the Icom 706, AmCom ASAC tuner, and a Stealth II 6m-160m screwdriver antenna already mounted in the auto.

Thanks for your comments.
73 N4GRN  
       

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K0BG
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« Reply #1 on: November 09, 2002, 08:26:43 AM »

I have run high power mobile since the early 70s, and I have done so with two different tube jobs, and three solid state ones. From my perspective, it is VERY difficult to go past 500 watts out when using solidstate devices. The nominal 12V systems in cars these days is the limiting factor. When GM introduces their 48V system next year, this will ease the problem "IF" they have enough alternator to handle the load.

Or, if you REALLY want to go high power (legal limit) you'll need a second alternator, three phase transformer, and everything else too. I have used four 4CX250B in grounded grid configuration and 1500 out is loafing. But let me tell you the rest.

Although the folks that make and sell the average screwdriver antenna tell you it'll handle 500 or 1000 watts, it won't. Oh it won't burst into flame like a hustler will, it will get HOT. Everything you do will have to be heavy duty from wiring, the alternator, extra battery (needed to do things right), and a variety of other additions. Quick and dirty just won't cut the mustard with high power.

Or you can maximize your whole setup, especially your antenna, and gain the same db increase but without the amp and all of the head aches.

If you want more input, I'll be happy to correspond via e-mail.

Alan, KØBG
k0bg@aol.com
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NX7U
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« Reply #2 on: November 09, 2002, 11:05:05 AM »

Alan is right on several counts.  There's a lot of extra beyond just the amplifier itself.  I would note however that the Stealth lineup seems more power-capable than most, judging from the loading coil size.
As to your question, I chose the SG-500 primarily because of the comprehensive failsafe mechanisms that are built-in.  It's physically larger and costs more, but for worry-free mobiling I don't think it can be beat.  
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Scott NX7U in DM43
N4GRN
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« Reply #3 on: November 09, 2002, 01:56:07 PM »

Scott and Allen;

Thanks for the info. I tend to over do everything in an effort to only do it 1 time. I hope to spend a lot of time in remote Labradore and Newfoundland over the next few years so I just want that extra punch when needed. So I was leaning toward the SGC amp as it seems to be top quality.

I will be installing an extra deep cycle battery (maybe 2 in parallel) at the amp with an isolation charging circuit from primary battery and alternator and possible a bank of large caps(the type used on auto stereo amps 3" diam X 12" long) as I have been told that they help a lot with peak power needs.

I have already inatalled a fused at battery #1/3 SO cord copper feed (+ and- both with a ground (not needed but there) in same cable) to the rear of the vehicle to a fused distribution block.

I was told that the Stealth's 4" diam coil of #12 wire and other beefy components should handle the power. I have installed 9913 coax thru a slieve into rear compartmrent where amp will be located. My matching coil at the base of the Stealth is a #10 coil very heavy. I have used 1" braid to bond everything except doors and etc where I had to use smaller size braid. My mount is a piece of 3/4"X3"X42" aluminum flat bar stock bolted in 4 places to the frame and also bonded with braid. The antenna feed point from factory is a 1/2" bolt into the stainless steel base.

Since I am not on HF yet I don't know how antenna does on transmit but is excellent on receive. I did make many contacts on 6m a few weeks back. I worked Cuba, Venezuala, Newfoundland, and many US locations from autu during several openings.

73  N4GRN
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K0BG
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« Reply #4 on: November 09, 2002, 03:42:31 PM »

George, you want to keep in mind a few basics. First, the method you used to mount the antenna is a common one, albeit lossy. The lower you mount the antenna, the higher the capacitive loading. It is not uncommon to measure a ground loss on bumper mounted 40 meter antenna at 12 or more ohms. Doing nothing else but moving it to the top of the quarter panel can reduce this by half! And half again when mounted on the roof (not always possible, obviously).

Higher than normal matching losses will also be present if you do not use a matching device of some kind. A good screwdriver antenna will have an input impedance of about 30 to 35 ohms on 40 meters. By adding a little inductive reactance to this (lowering the resonant point of the antenna) will bring this to nearly 50 ohms at the expense of added heat in the tank of the amp or rig as the case maybe. You can solve this with an unun or tuner.

And by ALL means, buy the fan kit for the SCG amp, especially if it is mounted inside a trunk.

As for the Hi-Caps, I use one on the supply line to the rig to keep the 706 from resetting due to the current inrush and susequent low voltage during starting. But at $100 or so each, you'd be better off with just another battery. I use a HD gelcel rather than a car battery as they typically have two to three times the current capability.

Good Luck!

Alan, KØBG
 
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N4GRN
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« Reply #5 on: November 09, 2002, 08:07:27 PM »

I know, I wanted to mount the antenna on center of roof (well at least much higher than I have it). However, the coil is actually at roof level since the base is 3' tall, then the 102" whip above that so I am already close to 10' above the ground with tip of the whip. I was told by the manufacturer that as long as the top of the coil was at the roof line I was OK. Is that correct?

The antenna has a Q of 450 and only uses 46 turns of #12 wire on 80-meters. It has a 7.2% radiation efficiency as opposed to less tha 1% on most other screwdrivers. It has a lexan coil and aircraft aluminum base and silver plated beryillum contacts on the coil and a pittman aircraft motor. All fittings are stainless steel.

Also I have been told by 2 people including the manufacturer that I can bend the whip down to the front of the car and may actually be better than straight up while driving? Any comments?

I do have a unun installed but was told that a shunt was better. I have both to play with what is your opinion.

My antenna has a massive quick disconnect so I am working on an extension pole of 2" aluminum tubing that can be extended so that top of coil will be about 8' above the roof for when I am parked or camping.

I really want the most efficient setup possible and since I was going to do this back in 1965 I am a little behind the learning curve so any help will be appreciated. Cost is a factor but a world class mobile rig is what I desire.

Thanks for all of the good advice.

73 N4GRN
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K0BG
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« Reply #6 on: November 10, 2002, 09:38:56 AM »

First off, calculated Q (theoretical) and actual Q (measured) are two different things. I'd be surprised if the coil actually measured a Q of 450. Typically, the figure is 250 for a good screwdriver, but some of them measure a lot less. The theory behind the screwdriver is that it is not short tapped which drastically lowers the Q. But rather the unused portion is covered by a sleeve and at the potential of the mast (we're back to theory and reality). The figure of 7.5% may indeed be correct for the upper bands, but not 40 or 80 where 2% is good! The scenario basically goes like this: You have coil, ground, and radiation resistance (there are a few more which can be ignored) making up the input impedance. The calculated impedance is based on the lenth of the mast and the length of the whip, and is very close to actual measurements. Then if the input impedance is higher (it will always be) then the additional figure is the losses experienced. If you measure a hamstick mounted on the bumper of the average car, it will measure about 65 ohms on 40. The calculated input is about 12 ohms due to its short length. That is a lot of extra loss and related to about 1 or 2 percent efficiency. In short, you should beg, borrow, or steal (maybe buy) a good antenna analyzer and measure the input impedance.

As for bending it over.. this can work, but usually doesn't. You're adding capacitance to the top of the coil which has the effect of shortening it, but just the whip alone won't provide enough capacity. I've used  "L" shapped antennas in an effort to make the length a full 1/4 on 20 meters (about 16 feet), but it didn't work as well as a shortened one with a coil. Look up my call here on eham.net for a picture of my antenna. It is just 2 feet short of a 1/4 wave on 20.

The unun is the best way to match because the shunt needs to be changed along with the band. The unun usually doesn't. Or, you can use a tuner. Most hams think tuners are lossy, and they can be, but an LC type like the Icom AH-4 or SCG units, run in the neighborhood of 94 to 98%..a very small amount. Fact is, properly done, you can use a 20 meter antenna on 40 with a tuner and equal or slightly better a resonant 40 meter antenna. Some will argue with this, but it just means they have never calculated or measured the real world.

I hope this helps, but rather than add insult to the forum, you can always e-mail me at k0bg@aol.com. I'll even give you my phone number or call yo on my dime.

Alan Applegate, KØBG
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NX7U
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« Reply #7 on: November 10, 2002, 02:29:50 PM »

Alan pretty much knows his stuff...
1. Coil Q's of 450 in the literature are nearly non-existent...250 is pretty darn good.
2. Yes, do whatever it takes to get a *vector* (that is, R+/-jX) impedance analyzer to look at your installation.  As one note:  I measured a 40m Hamstick antenna mounted on the rail (top of the wall) of my pickup, and got 18 ohms at resonance.  While this is much better than Alan's figure, it still is pretty inefficient.  
2a. But it's more efficient than nothing...
3. I use a shunt coil and it seems to do fine on both 40m and 80m.  There is a bit of pushing back-and-forth between the two bands but it works, and it's easy.
4. My HS-1500 measures 5 ohms on 80m and 9 ohms on 40m (obviously w/o the shunt).
5. Bending the whip over requires some care...it is similiar to a top-hat where you are adding capacitance to the top, but that also increases the current thru the loading coil, so there is a possibility that any increase gained by the top-load is nullified by the increased I2R coil loss.  A simple field-strength measurement would be handy.

I also have various pix of my install at http://members.cox.net/nx7u

Good Luck!
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Scott NX7U in DM43
NX7U
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« Reply #8 on: November 13, 2002, 11:43:55 PM »

CORRECTION:
I found my notes from 1999, and the measured base resistances of my HS-1500 are as follows:
80m 11 ohms
40m 21 ohms
Still not bad, but not so optimistic :-)
http://members.cox.net/nx7u/mobile/performance.htm
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Scott NX7U in DM43
WA4PTZ
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« Reply #9 on: November 14, 2002, 06:13:02 AM »

WHY?   I have talked and worked CW all over the planet
mobile with 100 watts. You can too. Besides...
the power requirements for the amp mean that you will
have to replace your alternator with a heavy duty ,
higher output one and replace it about once a year.
You will have to run at least #6 direct from the
battery (fusing it is another problem )and you may
even have to add another battery to the car. The high
RF field may reak havoc on the car's computer and
other items and vehicles as you pass by while
transmitting. And according to US standards you will
have to place warning placards on your vehicle due
to the high RF field.
But, if that is what you want to do....
73 - Tim
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N4GRN
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« Reply #10 on: November 14, 2002, 09:37:06 PM »

I have plenty to think about with all of the good advice given (and a lot to learn). I do have a vector analizer and an swr meter so I will be playing with differnt positions, whip lengths, and probably a cap hat etc., to see what is the best for my setup both while mobile and while parked.

I also wish to answer the reply about why the Amp. I do not plan on using the amp most of the time. I agree if my antenna system is maximized for my installation then I will only need the 100W's.

However, as I stated, I will be spending time traveling remote areas of Newfoundland and Labradore in winter months with -30f weather and ice and snow and at times many miles between villages. I would like to know that if an emergency did occur that I might have the extra power needed to reach someone if required. Cell phones have been of no use on past trips and many hours and days at times lapsing between seeing another auto.

Also I hope to do some DX while in those area that I could not do from Georgia so the extra power would be nice to have. I would certainly not be driving around running 500W. The amp would be solely for parked situations while camping or resting and probably rarely needed then as I hope to maximize my antenna system.

As for power required for an amp, due to the cold temps and remote areas, I will be having a backup supply for the auto anyway (solar alternate charging system, extra batterys, isolator, and etc). I have an electrical company and a general contracting company and have access to almost anything needed in the area of wiring components and for 10 years I had a used car business so auto parts and installation I can also handle with ease.

THANKS FOR ALL OF THE GREAT SUGGESTIONS. I now have much more to consider than when we started this discussion and a lot to learn. But learing and experimenting is a big part of what this hobby is about in my opinion. And Yes, I hope to get proficient with CW and not just learn enough to pass the test never to use it again. That is why I am taking my time learning CW.
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K8KAS
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« Reply #11 on: November 21, 2002, 03:29:01 PM »

As others have said, why in the world do you need a full KW mobile. I have worked all over the world with my IC706 (100 watts) and Tarheel Screwdriver antenna
mounted on a trailer hitch. The antenna base is about 24 inches from ground with a 6 foot whip on the Tarheel. High power can be (no, IS) total frustration from an EMC-EMI perspective and as well total vehicle power need and supply. Todays auto power systems supply about 1700 watts total. If you must I would suggest a second battery(gel film)in the trunk and short leads to the amplifier with a charging lead from the vehicle DC, this has worked well for me in a 500watt/amp system install. While most Detroit OE's test their vehicles(EMC/EMI) and (safety related systems) to a 200 volt per meter level, other automotive electronics are not tested to the 200 volt per meter value. Higher power will most likley effect these other systems such as power windows, door locks, interior lights, trunk locks, seat controls, am/fm radios, nav systems and on and on.  
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N4GRN
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« Reply #12 on: November 21, 2002, 08:49:27 PM »

I ASKED ABOUT DIFFERENCE IN QUALITY OF 1000W SKYWALKER AND 500 SCG AMPS. I NEVER INTENDED RUNNING 1000W MOBILE.

I also stated that the amp would be used while camping in remote Labradore and Newfoundland. NOT while driving.

I plan on some extended camping trips in remote areas during winter (30 below type weather NOT WINDCHILL TEMP). I also stated that I was installing backup battery system and solar charger for car backup and for amp while parked. I stated earlier that I have a #1 copper feed cables to the rear of the auto from the main auto battery with a fused distribution block located at the transmitter and amp site within 12". I also have battery isolation equipment, solar alternative charging, and will either be using gel or deep cycle batterys as alternate and backup source of 12v power for both auto emergency and radio.

I agree 95% of the time 100w with the Stealth II screwdriver is all I will ever need. I have it mounted so that while camping I can raise it to top center of auto on luggage rack and while driving located on side bar as earlier described.

This was intended as a discussion about user experience with two differnet amp manufacturers. The Skywalker and the SGC. The 1000w skywalker also has 6 meters that was the only reason for discussing it other than price difference. I simply commented that for less than 1/2 the cost of the SGC, Skywalkers 1000w could be purchased.

Thanks for the comments.
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