¦ Article ¦ HF-antenna's ¦ Center loaded mobile HF vertical ¦
Last review: 02-08-2005
On the road...
As we all probably know, traffic conditions become worse year after year while the distances between office an home QTH remain the same. So being stressed by traffic jam I seeked a way for a more efficiënt time use. Being forced to be "on the road" at least 2 hours a day (traffic jam excluded) I began to evaluate the possibility of mounting an HF antenna to my mini-van, it is a Mercedes Viano with a Common rail direct injection diesel engine. My colleague Jan, ON6ZG, inspired me with his new FT-857 and also a very fancy hustler antenna including the appropriate swappable coils. It's nice looking and seems to work fine. Another fellow HAM from work, Walter, ON4BCB, has a ATAS-100 which also couples nicely to his FT-100 from Yeasu. When portable he uses a whip as long as possible with an automatic antenna tuner from SCG. I have a Kenwood TS-50 without tuner which I want to use as mobile rig.
All antenna knowledge I obtained says me that the shorter the antenna the more I will have to deal with low radiation resistance and lower efficiëncy. Care should be taken to keep the losses in the system as low as possible.
Propagation and engine QRN are to be considered when going mobile. I choosed to go for 40/20 meter vertical. Quick release and easy mounting are one of the requirements.
A fixed length radiator has a feedpoint equivalent series circuit of a decreasing resistor and increasing capacitive reactance. To tune out the reactance we will need inductance ( coil ). The coil value will be determined by the placement and the length of the radiator. I consulted the 20th edition of the ARRL antennabook for following project.
Radiation resistance (Rr=Ohm).
Defined as the resistance that would dissipate the same amount of power as the antenna does with the same Rr. A 90° long vertical has about 36 Ohm at resonance. Resonance means that the antenna is purely resistive and this can be thru for a longer AND shorter than 90° radiator. For a less then 0,1 wavelength vertical we can calculate the radiation resistance from following equation :
Rr = 273.( l.f )² . 10-8
l = length of antenna (inches).
f = frequency (MHz).
Table 1 : Rr = f(Frequency)
|
|
Length(in) |
118,11 |
| Length(cm) | 300 |
| FREQ (MHz) | Rr (Ohm) |
| 3500 | 0,47 |
| 3600 | 0,49 |
| 3700 | 0,52 |
| 3800 | 0,55 |
| 7000 | 1,87 |
| 7100 | 1,92 |
| 7200 | 1,97 |
| 14000 | 7,46 |
| 14100 | 7,57 |
| 14200 | 7,68 |
| 14300 | 7,79 |
Knowing that we have to match to a 50 Ohm transmitter the Rr values are quite low aren't they ? Due to the low value of Rr, coil losses and ground losses will have a major impact to the final efficiëncy of the antenna.
Inductance (XL) for the radiator.
We have to compensate the capacitance due to shortening the radiator. A possibility of obtaining the needed reactance is the use of a "coil".
We can wind the coil fairly easy ourselves or use a commercial available one. I choosed to use my own construction techniques following some guidelines for obtaining high reactance to resistance ratio. (unloaded Q-figure ). If we use small-diameter wire and make a long small coil we will never obtain a Q-figure above 50. The resistive loss in those coils can be as high as 50 Ohm. If we want to make a high-Q coil we need to follow following guidelines as close as possible.
- Use large diameter wire.
- Diameter/length < 1/2.
- Air wound.
- No metal in the Coil.
- Large spacing between turns.
Only the last guideline can be improved in my design since the spacing is only a few millimeters .
Q figure versus resistive loss.
Coil resistance "consumes" power which doesn't benefit to radiaton, hence contributes to lower efficiëncy of the antenna. As the frequency lowers, the coil loss becomes more important.
Rc = XL / Q
Rc = coil loss in Ohm.
Table 2 : Approximate Rc values for 3 meter long verticals.
| Freq(MHz) | L (uH) | Rc (Ohm) | Rc (Ohm) | Rc (Ohm) | ||||
| Q=50 | Q=300 | Q=600 | ||||||
| 3,600 | 120 | 54,2592 | 9,0432 | 4,5216 | ||||
| 7,100 | 30 | 26,7528 | 4,4588 | 2,2294 | ||||
| 14,200 | 8 | 14,26816 | 2,3780267 | 1,1890133 | ||||
| 21,200 | 1 | 2,66272 | 0,4437867 | 0,2218933 |
Ground Losses (Rg).
Ground loss contributes to the total loss of the system by an amount which is determined by the groundplane of the car and the environment around the car. Some measurements showed that a groundloss from 2 to 15 Ohm should be teken into account. The total Resistance of the system is :
Rt = Rr+Rg+Rc(cos² h1)
with h1 the height of the coil
knowing that only Rr is radiating our power.
I measured my feedpoint impedance at 7.075 MHz with the AEA CIA HF analyzer to be 22+j2 Ohms.
Rr = 2 ( table 1 )
Rg =15
Rc=5 (table 2 )
VSWR and bandwith.
On both 20 and 40 meter the vertical has about 20 Ohms at the feedpoint about 40 KHz bandwith on 40 meter and 200 KHz on 20 meter. The low feedpoint impedance requires a matching for connection to 50 Ohm transmitters.
If you ever see a mobile vertical advertised without matching device it's almost certainly due to the use of low Q- coils, those verticals have very low efficiëncy but will sell much beter because some hams like low WSVR figures. A 50 Ohm resistor has a 1.0 VSWR but doesn't radiate much of your power doens't it ? Always look for large coils and heavy gear, avoid the whips with a coil as small as a pencil.
Efficiëncy.
The lower the frequency the lower the efficiëncy of small verticals. I entered some formulas into an excel spreadsheet. Feel free to experiment with the values of Q (coil ) frequency and length of the radiator. You will see that one need a very good coil to have about 10% of efficiëncy on 40 meter. On 20 meter it is possible to obtain almost 30% efficiëncy with a Q of 300, while a coil with a figure of 50 has about 20%. So, go for the high Q and use a matching circuit to transform the impedance to 50 Ohm. Don't concentrate TOO much on VSWR values as they don't guarantee a good radiator.
Impedance Matching.
If your vertical, mobile or fixed ever has an impedance which is 50 Ohm don't be too happy, you might have a bad grounding system or to much loss in whatever shortening system. The above project has about 20-25 Ohm on both 20 and 40 meter, we can use a coil as Betamatch to transform the impedance to about 50 Ohm. I preferred to use a 1:2 broadband impedance transformator to transform any impedance from 2-30 MHz to 50 Ohm. Making more coil for the other bands is all we need to do to QSY the mobile to other HF bands.
A trifilar trafo on a TX 36/23/15 - 4C65 core.The u=125 and AL=170. Available at www.dx-wire.de. I used 3 windings for this trafo. More info at ARRL antennabook 20th edition, page 11-43.
ON THE AIR.
I made several contacts both on 20 meter and on 40 meter. The car engine produced some QRN which could be handled with the TS-50 noise blanker. It is fun being on the air with such a mobile antenne. But hey guys, I surely wish there will be more HAM's driving around with those kind of structures on their cars because I need your help in explaining people what we're doing and why...maybe some day then people won't see me anymore as the weirdo next door (hi x3)
CONCLUSIONS.
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This project will provide you with a good antenna to work mobile. It will be even better than the small coil commercial ones. With limited mechanical skills you can build one yourself.
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The satisfaction is all yours if you work the ultimate DX with your homebrewed antenna-"construction.
SOURCES..
1/ARRL Antenna book 20th edition
5/EZNEC 3.0 from W7EL/ www.eznec.com
6/Hamcalc from VE3ERP.
