ANTENNA/BALUN/POWER OUTPUT, I'M CONFUSED
I'm running a yaesu ft 450 into an LDG 200 Pro, through a MFJ watt/swr meter, an LDG 4:1 balun, then into a MFJ 1777 doublet in an inverted v configuration with (the apex at about 40 feet and ends about 25 feet below the apex) 80 ft of 400 ohm ladder line. I communicate very well on 80-10 meters (I'm afraid to try 6 meters). However, on certain bands the swr gets high and my radio rolls back to 60 watts output. This doesn't bother me as the Ldg ATU power/swr meter, and the mfj power/swr meter correspond exactly (as far as I can tell) and, I make good contacts all over the world. My understanding is that the radio is protecting it's self against a high swr, and with a multiband antenna there has to be a trade off somewhere, right? I've made plenty of world wide contacts on 60 watts, and the highest SWR I've seen is 3:1.
What I don't understand is that I've been advised that I should be using 1:1 voltage balun. However, I've also had the 4:1 current balun recommended. I'm a newbie, and confused. Can someone calirfy for me? I've considered building a dual purpose 1:1, and 4:1 switchable balun, but the ones available are not for use outside the shack (not protected against weather), and since I have to push a button to switch between the two I'd have to interface the balun right off the ATU, and I don't want to do that because I'm concerned about RFI inside the house.
May I have your experienced opinons please?, and thank you in advance.
Would you clarify a few things?
1) Is the MFJ meter between the rig and tuner or is it between the tuner and the balun?
2) When you talk about the LDG and MFJ meters corresponding exactly, does that mean that they show the same SWR value? same power output?
3) Is the LDG a voltage balun or a current balun?
My experience with automatic tuners is that you may have to fiddle with the feed line length to get the SWR below 2:1 on all bands. Even then there is no guarantee you will be able to do so. A manual T network tuner will generally match the full range of impedances on an all band dipole like yours. The Ameritron ATR-30 is an excellent one. For low power and minimal cost, I also like the MFJ-949E.
However, the convenience of your automatic tuner may be worth the reduction in signal (60 watts is only 2.2 dB down from 100 watts - less than half an S unit) for casual operating.
From the reading I've done, a 1:1 current balun is probably the best choice for your setup. For some impedances, the 4:1 might be better, but for others it can be much worse (e.g. converting a marginally low impedance to a very low one). In any case, you want a current balun, not a voltage balun for your situation.
I wouldn't shy away from trying 6 meters. Your setup is not ideal for it, but it will make contacts. I have used my 67' dipole fed with balanced line on 6 meters to work stations 2,000 miles away when conditions were good.
Originally Posted by KJ4TOF
I have had to download the manual for the MFJ1777 to understand what you have. It might have helped if you elaborated a bit. It seems to be a 100' dipole, centre fed and supplied with 100' of 450Ω line. This is essentially a G5RV with open wire feed (though no doubt some will argue the toss on what is a G5RV). You are looking at a configuration dealt with at Fig 12 at Feeding a G5RV though the results will be dependent on actual feed line length.
The balun you are using is IIRC a voltage balun. Voltage baluns are often used because of anecdotal evidence that they make an antenna system easier to match up. That is probably because voltage baluns are pretty lossy when subjected to the high differential voltages that occur with the extremely high impedance that can occur at times with this type of antenna system. Keep in mind that ferrite cores are very slow to heat on continuous carrier, more so on SSB, so if the core is warm at all, it hints that there is significant loss. If you replace it with a current balun, you might find it even harder to match, and you might be convinced that current baluns don't 'work'.
But, the underlying problem is probably the occurrence of extreme impedances on some frequencies, and the ATU is unable to properly transform them to close to 50Ω.
Have you tried increasing or decreasing the feed line length a bit. This can be a bit tedious finding a length that matches well on all bands.
If it were me, I would be using a 1:1 current balun designed for high common mode impedance and high voltage withstand. I would then play with the feed line length. This should lead to something that works acceptably over the HF bands. Your target with the ATU should be VSWR less than 1.5. (Note: you must not coil up excess open wire line.)
Regarding switched ratio baluns, the switches used cannot usually withstand high voltage, and they become a weak point in the system.
I hesitate to offer any suggestions because as others have pointed out your information is incomplete.
For an antenna system using a dipole type antenna fed with window line I would use a 1:1 current balun on the coax end of the coax feedline where it connects to the window line. The window line is acting as a transformer changing the impedance at the end of the line with the frequency bands. As Owen has mentioned finding the right length of window line to give an acceptable VSWR will "be a bit tedious finding a length that matches well on all bands", not impossible just difficult.
I'm sorry! I should have included more information. The MFJ 1777 is 102 feet wire dipole with 100 ft of 450 ohm line feed (which I have reduced to about 80 ft). I have 2 ft of LMR 400 coax connecting the radio to the SWR meter's input, 1 ft of coax (that came with the LDG ATU) from the swr meter to the ldg tuner, then a 6' LMR 400 coax from the tuner to an MFJ window feed through unit. On the outside of the house I have a 3' piece of LMR 400 connected to an LDG 4:1 current balun, then the 450 ladder line connected to the balun. When I said the mfj meter matched the ldg's power and swr meter I meant that when I xmit they both appear to give the same indications as to my output power and the swr. None of the ladder line is coiled up. However, one of the trees I had taking up the excess slack has been blown over. This means that I will either shorten the ladder line another 10-20 feet, or snake it around my yard with a 6' piece of wooden stake (with a glass insulater of course). By the way, that raises another question; is wood considered a conductive material? I hope I provided enough information. The thought of altering the feed line lengths had not acurred to me. So, are you folks saying that it is possible to get a 2:1 ratio across the 80-6 meter spectrum with the right tuner? How hard is it to work a manual tuner?
tks for the feedback!
Last edited by KJ4TOF; 12-21-2010 at 03:16 PM.
EZNEC says the 3.8 MHz impedance seen at the ladder-line to coax junction (where the balun is) equals
Originally Posted by KJ4TOF
"Impedance = 7033 + J 3404 ohms". That is an impossible impedance for any balun including a 4:1 or a 1:1.
If you go back to the ladder-line length shipped with the antenna, that impedance drops to
"Impedance = 95.52 - J 789.8 ohms" which probably will work with either a 4:1 or a 1:1 balun/choke.
If you want 3.8 MHz operation to work well with a 1:1 balun, increase the ladder-line length to 120 feet where
"Impedance = 30.82 - J 257.1 ohms".
Moral: One cannot just willy-nilly choose any random length of ladder-line for a multi-band HF dipole.
One can model the above 3.8 MHz conditions (with reduced accuracy) using the free demo version of EZNEC available at www.eznec.com.
Last edited by W5DXP; 12-21-2010 at 03:22 PM.
73, Cecil, www.w5dxp.com
The maximum power transfer theorem works just as well for a non-resonant antenna as it does for a resonant antenna.
I thought I read that the angle of the inverted V would have some effect on SWR , You say that the end are 25' below the apex , how close to ground are the ends , maybe an issue also .
I made a mistake, the balun is a voltage balun. Not a current, I could have sworn it was a current, but I was wrong.
The ends of the wire dipole are only about 16 feet above ground, but I have no choice with the size lot I have, except to go with a smaller antenna, but I hate to give up 80 meter band. I guess I could install some traps but I have no idea how to install them.
By the way, I've been told by local club member that soldering more ladder line to lengthen it would be a bad idea. I'm just telling you what I've been told, and these guys have been into it their whole life. That's why it's all so confusing to me.
Thanks again for your input.
Silver solder the connections, then it won't be a bad idea. Don't forget to weather proof the joints as well.
73, Dave ACØYW
Cecil, you said "any balun". Can you explain why you say that in respect of a 1:1 Guanella balun?
Originally Posted by W5DXP