Yaesu FT-7800 reviews about low output power

[KE5TJT]

I am really questioning whether people know how to check output power on a mobile HAM radio. You have to hook up the radio properly to the SWR/power meter and ensure the SWR meter used is for the correct frequency. You then must set the radio to high power. Key the mic and observe the needle.

But wait.....a VERY important step was left out. Ensuring 13.6 volts to the radio!!! A car battery at rest is NOT putting out enough voltage for a full 50 watts output!! My radio will only do 30 watts out on high with the vehicle not running. But when the car is started, or the battery is plugged into a charger, suddenly I get the full 50 watts! I have a feeling people don't know what they're doing and think that just plugging it in and keying the mic should get you 50 watts out, it's not that simple. It gives the radio a bad rap when there only problem is the end user not knowing what they are doing. My old VHF marine radio is the same way, 14 watts on high with engine off, 25+ on high with engine running or battery charger plugged in. When you see the technical specs showing 13.6 volts, that's what they ran the radio at to get the max rated output power. That doesn't just mean "a 12v system" it means AT LEAST 13.6 volts.

[K5LXP]

The 7900, and most any contemporary radio is spec'd for 13.8V DC input, plus or minus 15%.  It *should* put out rated power down to 11.73V.  That means 11.73V at the back of the radio, not at the end of the power cord. 

If we run the numbers:

T9021715 Yaesu Power Cord  14AWG, 9ft/Pair, 18ft/single

14AWG=.002524*18= .045432 ohms

TX@8.5A = .386172 volt drop.  Figure about 5 milliohms of internal impedance for a fully charged SLI battery, so .0425V of drop at the terminals.

.386172+.0425=.428672 volts of drop when transmitting on a 7900.  The resting voltage of a car battery is nominally 12.78V, so 12.78-.428672=~12.35V.

That is plenty of headroom for a radio spec'd to 11.73V.   With the engine running there's a good volt to volt and a half extra which could easily make up for a slightly resistive termination or fuseholder.


Mark K5LXP
Albuquerque, NM

[KE5TJT]

Where did you get 8 amps from? 50 watts / 12 volts is 4.1 amps. Even with the radio on, I think it draws no more than 5.

[VE7DQ]

Most VHF radios are only about 50% efficient; it takes about 100 watts @ 13.8 volts to produce 50 watts output.  The other 50 watts are dissipated as heat through the heatsink on the radio.  That's why they draw 8 amps or more, and why the heatsink gets warm if one gets long-winded on transmit.  8.5A is an appropriate number, and any inexpensive volt-ohm-ammeter (most will handle 10A) will illustrate that.

Regarding your HAM radio; it's not an acronym, so it isn't capitalized... It's just a ham radio, just as you and I are ham radio operators... unless the word 'ham' is the beginning of a sentence... then it would be 'Ham'.    

Cheers and 73
Tom

[M6GOM]

Where did you get 8 amps from?

The specification table in the user manual.

50 watts / 12 volts is 4.1 amps.

Doesn't work like that. You're assuming 100% efficiency from the PA and every other component in the radio which it isn't.

[KE5TJT]

I see said the blind man.

[KE5TJT]

I have some unused "holes" in my fuse box. I am going to put this on a fuse. I guess 10 amps would be plenty if you say it draws 8 at max, right? It's got its own fuse, and if it blows the 10 amp I can always just put it on a 15.

[K5LXP]

I would use the same fuse the radio has, which for the 7800 is 15A. 


Mark K5LXP
Albuquerque, NM