Antenna Efficiency Calculation or Gadget?
I just installed my mobile HF antenna system. It consists of an Icom AH-4 auto-coupler, a 4" length of #8 wire to the threaded 1/2"rod of a ball mount. Antenna is a simple 102" stainless whip on a spring. I will work mostly 100W SSB
I know from prior experience that this setup works 'well enough' on 20-6 meters. I realize that mobile installations are definitely a compromise!
I suspect, from simple formulas and guidance (k0bg.com) that on 40 meters, my efficiency is suspect (20 even!) (I did make one QSO from NH to GA last night on 40), and on 80, all/most the power is radiated in the coils of the coupler! (I did make a short-hop to VT 5/9 QRP)
I am now wanting to improve my efficiency. I have to stick with my long whip due to wind loading on very cheap sheet metal on a Hyundai. I mounted the custom-made ball less than 1/2" from a seam on the vehicle metal for added strength, but a screwdriver is out of the question on this vehicle. I am sticking with both vehicle and existing hardware!
So, I'm asking if there is a way to gauge efficiency through the coupler, on various frequencies? Something like an antenna analyzer? Will something like this work through a coupler?
I'd like to improve ops on 40 and maybe get something useful out of 80. I'd like to experiment with adding temporary length to the whip and see the results, etc.
Any help appreciated!
73 de KA1YBS
Not an easy task.
I would determine the radiation resistance (Rrad) of the 102" whip for each band (by calculation or modelling), and then measure the current into the bottom of the whip using something like a clip-on RF ammeter. I*I*Rrad will then tell you how much power is being radiated. Knowing the power from the rig into the tuner, you can calculate the overall efficiency. I think you'll be surprised how little it is on the lower frequencies.
Here's the EZNEC Rrad predictions for a 102" 1/4" vertical:
80m 0.4, 40m 1.4, 30m 3, 20m 6.4, 17m 11, 15m 17, 12m 26, 10m 39
You might want to try what I did. I parked my car at a convenient and desolate location (so to avoid busybodys), and using a string attached to the antenna mount, draw a large 20 foot circle around the car. I then used a simple RF Field Strength meter at fixed locations around the circle to determine the relative field strength around the vehicle body. It is important that the field strength meter be in exactly the same position at all times when you change something in your setup. Oh, and bring a pair of binoculars to read the meter. Standing next to it will give incorrect readings. The field strength meter should be at a convenient height mounted on a non-metal stick or fiberglass pole.
This will give you some idea of the field strength when you make an installation change whether good or bad. Remember this is only for relative readings and higher readings do not necessarily mean a stronger signal. It could just mean your radiation angle has changed. The way to check that is to take 2 measurements at different heights. Not you, the FS meter. Hi Hi.
It is cumbersome and lots of work but it may pay off for you.
Last edited by K2WH; 04-12-2009 at 05:36 PM.
I've used an AH2 and 102 inch whip myself. I ended up confining its use to 20 (later also 30) meters and higher frequencies, because it just plain doesn't work on 40 and 80 (or the MARS frequencies there). Physics won't let it. For 80, 75 and 40, I played with a Hustler on a rear quarter panel and even a Bug Catcher on the roof. (Hustler on 40, Bug Catcher on 80, whip on the others. Rig in the glove compartment of my '87 Chevy Sprint. All that and almost 50 mpg!
You COULD add a base loading coil under the whip. That might not be as good as even a Hustler (better than you'd think on the roof, though) , but might be good enough. Get a Bug Catcher coil with a tap lead and you can adjust it for 80 or 40 as needed.
BTW, put a stiff doubler plate under the sheet metal and a twin above it. You'll want weather gaskets (I've used cut up truck tire), and it's best if the screws/bolts go through them. But that will spread the mechanical load and make it safer to mount a good sized antenna on the tinfoil car makers use nowadays for good mileage.
Added: There is a TON of information on the Web. Try http://www.w8ji.com/ or http://www.k0bg.com/ .
Don't forget, a great deal of the loss in a mobile is the ground under and around the antenna and the car. With a bad enough ground, a perfect loading coil won't help much.
Last edited by KA5S; 04-12-2009 at 06:59 PM.
For a "good" screwdriver antenna (full sized one, like the Hi-Q etc) nobody mounts these directly to their car sheet metal, no matter what kind of car it is. Not strong enough.
Everybody bolts or welds a bracket to the chassis or frame *under* the vehicle, and makes a bracket to support the load of the screwdriver. Most use a secondary bracket up higher for additional support, even when installing on very heavy and strong SUVs and such. The Hyundai would be no different.
Based on the Rr of the 102" whip on 40m, its efficiency is about 3%. It's much worse on 80m. That doesn't mean contacts cannot be made. I've worked Florida with 30mW, that's 83,000 miles per Watt. Many have done far better than this...there's already a "million miles per Watt club."
I worked Ohio using a 60W GE Soft White light bulb as an antenna, while running 40W output from a DX-60B. I imagine the "e.r.p." of that setup was waaaay less than one Watt.
When propagation and the other guy's antenna and receiver are doing 99% of the work, it's amazing what can be done.
Still, if you want to make more contacts, easier, use a different antenna, especially for 40m and 80m.
Thanks for the replies!
I'm a bit concerned that perhaps a large whip is really insufficient
I absolutely love the idea of the coupler working with the radio while mobile, it's safe and easy to band-hop with. I know advanced screwdriver antennas also offer the same convenience, albeit with more efficiency and size. Unfortunately, my budget and current mounting system just won't allow them.
I am now wondering if using a base/center loading coil is appropriate, while keeping the coupler in-line? I am thinking that the coupler has a set of fine coils already in place, and perhaps there will be some reactance between them...
One more, is it a waste transmitting 100W into the existing system, or is the power to efficiency ratio 1:1?
Many thanks again. Part of the fun here will be learning and optimizing.