Discussion in 'Ham Radio Discussions' started by KC3EPA, Apr 15, 2018.
Haha let's try again. 100W transmitter, 2:1 SWR antenna, and a perfect tuner.
How many watts go out into the ether?
Attempting to load a mobile antenna below its' resonant band is a poor choice. It will develop high voltages that may damage your tuner.
This is not a case of a poor advertising or false specifications.
Not enough data to say. You can't determine efficiency based solely on SWR and original incident power. It's funny that both the Amateur and Commercial FCC element question pools have misleading questions on this subject, and so there are tons of people who have learned it the wrong way.
"Power into the ether" depends on the whip's intrinsic efficiency (which depends on length, material, size, orientation with respect to the vehicle body, orientation with respect to the ground, and its overall shape), the ground losses (which includes the car body material, its shape, its proximity to the ground underneath it, the ground constants of the ground underneath it, the quality of bonding between the car body parts, etc.), and the length and loss factor of the coaxial cable being used to feed the mismatch. If a tuner is involved, it also contributes to the feedline losses, because the L and C elements aren't perfect, and they all have losses (in the case of some tuner vendors, the L elements are very lossy due to the core materials selected).
If a shunt matching device is used (see K0BG's site for an example) at the antenna to raise the feedpoint Z, it also has losses, and if the device has insufficient L for the frequency used, its losses can also be substantial.
A 2:1 mismatch at the antenna will imply a very specific reflection coefficient, but the losses associated with it are very dependent up on all these other factors I described, plus some others I have probably neglected. The mismatch itself does not introduce any losses, but the effects of the mismatch can cause increased losses in other components like coaxial cable and tuning elements.
Again, not quite true.
You can match an electrically-short mobile antenna at its feedpoint without any voltage elevation issues on the cable. The whip itself my have higher voltages present, but a proper mount selection will mitigate this, as well.
There is nothing wrong with loading a short whip for mobile operation if it is done correctly. Military, aviation, and maritime services have done it with great success for decades.
I'm not sure what you mean by "minimum"... Minimum loss?
WAAAAAAAHHHHHH! don't get me started about loading up lawnchairs, bedsprings, t-shaped clothes poles, chain link
fences, towers, or barbed wire fences!!!
gotta admit, though, they look pretty funny in the ant____________ space on a qsl card!
That depends. Is the feedline also perfect?
Keep loss to a minimum by using quality components. If you can't buy them build them. I can't think of an off the shelf tuner that will handle a 4 ohm load without a lot of loss. It may not be an issue for qrp.
To put it simply, you may show a 2:1 SWR, but in fact have so much ground loss that it's actually masking the reality of the loss.
Tune it up with the car parked in a sandy area.
Now, check the same tune on pavement, steel reinforced concrete highway & other different conductivities of ground.
You will be pretty shocked to find a real poor SWR on the steel reinforced concrete, but if you retune, you will do much better than other areas.
Your car capacitively couples to "Ground".
When you are in different ground conditions, your percentage of watts wasted will be different.
To test & tune my antennas, I went up on a high hill that used to be a Nike site. Then, it was an antenna test facility.
300'X300' of concrete, with steel I beams exposed every 4', in a cross pattern. Yeah, I understand it's condos now, but there was no better place on the coast of MA, for man made ground.
In my youth, I remember watching them weld the base of a 150' antenna tower to the I beams & construct it in 2 days. I watched from the woods. I think I was 12, when I first thought towers were cool.