Pages: [1]   Go Down

Author Topic: Reducing Power on SSB to Make Batteries Last Longe  (Read 4988 times)

KE4SKY

  • Member
  • Posts: 1045
    • homeURL
Reducing Power on SSB to Make Batteries Last Longe
« on: November 06, 2001, 12:09:19 PM »

I would appreciate any comments on low power SSB operation in response to my post in the QRP forum.

I'm not convinced that the QRP rigs such as the FT817 can be very effective for Emergency Communications, when most EmCom is "short path" which requires mostly use of 40 and 75 meters, which are noisy and a weak signal may not even be heard through the ambient noise.

In my experience I have found that 25 watts on a full size wire dipole or G5RV is OK in good conditions, with a signal loss of only an S-unit compared to running 100w.  When running mobile with a hamstick, 25w may not cut it, especially during periods of high solar activity.

It's great to have a tiny, portable rig, but I'm beginning to think that for EmCom a compromise HF rig is needed which is reasonably portable, such as an IC706, but which can deliver 100w when needed.  If the band conditions are good you can always turn the power down to save your battery, and running minimum power to maintain reliable contact IS good amateur practice!

Any comments or discussion?

73 de KE4SKY
Virginia State RACES Training Officer

Logged

AB8JC

  • Member
  • Posts: 64
Reducing Power on SSB to Make Batteries Last Longe
« Reply #1 on: November 07, 2001, 12:43:04 PM »

This is a topic of ongoing discussion on both the HFPack and FT817 Yahoo! eGroups.

How about looking at the military HF rigs?  20W for portable HF SSB, 200W for mobile; both into autocouplers.

On the other hand, how often do we need HF SSB while mobile?  What I've seen needed while mobile was mostly local commo, more suited to VHF, than statewide or regional.  Section EC's and the like might need great HF mobile setups, but I'm not convinced that's the case for most hams.

Don't forget the receive power drain difference between the FT-817 and something like an Icom '706 -- a few hundred milliamps vs. 2+ amps.  That '706 will deplete a regular automotive battery pretty quickly, even on rx-only, while an '817 & an amplifier will last a looooooong time.
Logged

KE4SKY

  • Member
  • Posts: 1045
    • homeURL
Reducing Power on SSB to Make Batteries Last Longe
« Reply #2 on: November 08, 2001, 10:24:49 AM »

The military rigs aren't commonly available and are expensive for the "average ham."

We have tried to encourage quasi-standardization among our Virginia RACES operators who routinely do mobile and portable HF.  Of the common rigs out there which seem to have a proven track record, the most popular seem to be the various iterations of the IC706, the Yaesu FT900CAT (my personal favorite) and the older Kenwood TS430, 440, 450 series, all using auto tuners.

Some comments from Steve Cuccio, NB30, our Old Dominion Emergency Net Manager:

"These guys who think they can operate SSB with 5 watts AND a mobile antenna simply haven't been on 75 enough.  Generally EmCom and NTS operation requires a better signal for reliability than the normal CQDX UR 599 style QSOs.  If a person is one S-unit below 100 watts and a dipole because he is running 25 watts to save his battery, that's livable.  If he's two S-units down by running 25 watts and a portable hamstick dipole, copy becomes marginal.  If he's four S-units down by running 25 watts on a single vertical hamstick on a vehicle, you may not copy him at all unless he changes to CW and hope the station at the other end has a really narrow filter, especially with the splatter we have to live with on a crowded band."

Your mileage may vary, but I value Steve's observations, as they agree with my own experience.

I agree that QRP on 14 through 28 MHz works really well, but on frequencies needed for reliable "short path" on 40 to 75/80 meters, the noise level increases the lower in frequency you go.  On 160 meters, which we occasionally need to get a reliable late night winter path, less than 100w on a full-sized dipole is extremely marginal during periods of high SFI.

73 de KE4SKY
Virginia State RACES Training Officer
Logged

AB8JC

  • Member
  • Posts: 64
Reducing Power on SSB to Make Batteries Last Longe
« Reply #3 on: November 20, 2001, 02:49:37 PM »

The reference to the military rigs was to their power levels -- 20W for /P & 200+W for /M -- not to imply we should actually use them; they are overkill (unless, of course, one happens to have that kind of $$ just lying around......)
Logged

NO9E

  • Member
  • Posts: 1124
Reducing Power on SSB to Make Batteries Last Longe
« Reply #4 on: April 26, 2002, 03:16:21 PM »

Reducing the current drain also increases the optimum impedance of the radio. So while the optimum impedance at full power is 50 Ohms, the optimum impedance at 50% current is 100 Ohms and at 25%current is 200 Ohms. In other words, reducing the supply current of a 100W radio from 20A to 5 A, would yield approximately 6 W with a 50 Ohm antenna, or 25W if the antenna is matched for maximum output power (i.e., at 200 Ohm).

There is a limit to this reduction as a rig draws some minimum current without any signal. For a 100W rig, this could be 4A.

Reduction of max power makes only limited sense. If one sends CW where the radio transmits about 30% of time, the curent is 20A when transmitting and 4A when not, the average transmitting current is 0.3*20A+0.7*4A = 8.8A. When one limits the peak curent to 25% and matches optimally, the average transmitting power is 0.3*5A+0.7*4A =4.3A. Thus 2 times current reduction is at the cost of 4 times lower peak power.

AT SSB, the average current is even lower. It would be 5-6A without speech processing and up to 10A with bvevry good processing (RF).
Logged
Pages: [1]   Go Up