Doublet Troubleshooting

[Ve3dgy]

So at John’s suggestion I put up a doublet today, about 40’ up and V’ed down 30 degrees to two trees. I used about 55-70’ of ladder line to my MFJ versa Tuner III over to my Yaesu 857D. I used 12 gauge insulated wire, it was touching tree branches in a couple of locations. 

To my surprise after doing the “rough” tuning by ear(max noise) I was receiving brothers in the US no problem (I monitored the frequencies from the waterfall on my IC7300 to locate good frequencies to listen to.) But:

I’m no expert on the MFJ tuner beyond my uTube training but what it was telling me was this:
1. At 100w output the meter showed about 10 watts going out and maybe 1 w reflected, couldn’t read an SWR(would have been high where the two needles crossed).
2. The radio’s SWR meter was showing an SWR of about 2:1
3. The radio’s PWR meter was showing about 30watts output
4. It was blasting the input of my IC7300

I CQed a few times, no response. I don’t know if I’m transmitting or not. The uTube showed the meter going up to 80w and reflected about 20 and a decent SWR, I can’t tell what’s happening with mine. But it receives crystal clear, doesn’t that mean a low SWR and that the output power should be high?

I checked the 857D manual and it will restrict output power if it detects a bad SWR.

So what might be happening here oh gurus of the airwaves.

[Rob_Walker]

Doug, how does your ladder nine connect to the tuner?  Does the tuner have balanced line terminals or are you using a balun?

73 Rob VE3RWY

[Ve3dgy]

Rob, the tuner has balanced line terminals and I think  a 1:4 balance internally

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1:4 Balun.

[Rob_Walker]

Rob, the tuner has balanced line terminals and I think  a 1:4 balance internally

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1:4 Balun.

I'm not sure what else to say.  The balanced line input on my PalStar AT2K tuner was an S0-239 so I had to supply my own balun.  I first tried a direct connection and then tried a 4:1 BalunsDesign.  In the end I found a 1:1 balun worked the best.  I also played around with the length of my ladder line and ended up taking out 7'.  My PalStar tuner could never tune down close to 1:1 on most frequencies.  I was quite disappointed for such a good quality tuner and I also had a lot of noise.

I then tried my Ten Tec 238B tuner that had terminals for wires.  This tuner was amazing and I could tune tune down to 1:0 or very close to 1:0 on most frequencies.  You were right about being able to hear - it was amazing.  I tried talking to Dave G0EVY in England with it while running barefoot and he gave me a 44.  It worked well for nets along the eastern part of the States.

Just to be sure, say you want to TX on 3.783 MHz, do your put your rig into CW mode, turn the power down, key up and then physically tune the antenna first before going back to LSB and turning the power up?

73 Rob VE3RWY

[VA3KOT John]

My suggestion is to check two things:
First; are the two sides of the antenna of equal length? This is meant to be a balanced antenna so each half of the dipole should be the same length within a couple of inches.

Second; check the total length of each side of the antenna plus the feedline. If it is a half-wavelength (or multiple) on any band you are using there will be a very high impedance at the tuner that may be outside its tuning range. If that happens, as Rob suggested, try shortening the feedline.

Also check the ladder line doesn't come anywhere near anything metallic (soffit, fascia, eavestrough etc)

If you can't locate the problem I could come to your place, if you wish, to help troubleshoot.

[VA3KOT John]

Another thought:
Rob mentioned this already, but you should tune in CW mode because you need a carrier to measure. When you are in LSB/USB mode the needles on the tuner are only showing the amount of modulation which obviously varies as you speak into the mic. My MFJ tuner has a switch to measure either peak or average forward and reverse voltage on the cross needles. I leave it on the average setting so the needles only move slowly as I speak. 

The correct tuning procedure is to lower output power (5 watts is enough), set to CW mode (plug a key into the radio if needed), adjust the tuner's inductance for maximum noise, then adjust the variable capacitors (they are interactive) for lowest SWR. The SWR can be read at the point where the needles cross.

I have built a simple circuit that adjusts the ALC voltage (I'll write it up for the February newsletter). When I flick a switch my Yaesu FT-897 is set to 5 watts and transmits a carrier for tuning. When I turn the circuit off the power and mode return to their previous setting. Some radios are smarter than the FT-897 and don't need that circuit.

Going back to my previous reply concerning antenna/feedline length; if each half of the antenna is cut for 80m it is going to be probably 65-70 feet long. If, as you say, you have 55-70ft of feedline it is quite likely that the overall length will be close to 130ft and that would give you very high impedance on 80m and its harmonics (40m, 20m, 15m, 10m) resulting in high SWR. Try measuring the SWR on every band and see what you get. then you will have an idea what needs adjustment.

Note: The impedance at the end of a half-wave wire is 2500-5000 ohms. Even with a 4:1 balun the impedance only comes down to, at best, 700 ohms. The tuner still has a 700/50=14:1 mismatch to deal with. We need to get the impedance down to, at most, a few hundred ohms before the balun. Let's say the impedance at the end of the feedline is 600 ohms. After the balun, that reduces to 600/4=150 ohms and the mismatch is now 150/50=3:1 which is easy for the tuner to handle.

[Ve3dgy]

OK, thanks for the suggestions. I was transmitting on low power in LSB, the radio said I had an SWR 2-3:1, but the needles on the tuner were barely moving. I guess indicating a super high SWR. I roughly measured my feed line and it’s about 55’, in the danger zone I guess. The experts suggests 80’ is a safe length. I’ll add 25’ loosely coiled in the house and see what that does, and tune on CW. The ladder line does cross a drain pipe perpendicularly, about 3”, also the dipole (65’ on both sides of the feedpoint) brush against some branches, the wire is insulated.

[VA3KOT John]

OK, thanks for the suggestions. I was transmitting on low power in LSB, the radio said I had an SWR 2-3:1, but the needles on the tuner were barely moving. I guess indicating a super high SWR. I roughly measured my feed line and it’s about 55’, in the danger zone I guess. The experts suggests 80’ is a safe length. I’ll add 25’ loosely coiled in the house and see what that does, and tune on CW. The ladder line does cross a drain pipe perpendicularly, about 3”, also the dipole (65’ on both sides of the feedpoint) brush against some branches, the wire is insulated.

Doug, my MFJ-949E tuner has a 300W/30W switch; maybe your tuner has the same. If you are operating low power try the 30W setting then the needles will move further. The meter only indicates a high SWR when the needle on the "Reflected" side has a high reading. If the needles don't move very much at all it only means the tuner isn't seeing enough power from the radio. Again, see if your tuner has a low power setting. And don't try to measure SWR in LSB mode, use CW.

Don't worry about the antenna wires touching branches; it doesn't matter at HF as long as the wire is insulated, as you say. I don't recommend coiling the excess ladder line. You can do that with coax because the coax braid is grounded. Ladder line has a voltage on both conductors so you may get unpredictable interactions if you coil it. Better to route it round some trees or poles outside. Also, don't let the ladder line touch the ground.

[Ve3dgy]

Well gents it looks like I may have tamed the tiger. I added 25’ and managed the SWR to 1.2:1, on 10M and 80M. I have to collect the data across the bands to really see what I have. Will try a few CQs on 80M but the real test will be the Net on Wed, see if people nearby come in stronger. Funny a month back I had no trouble on the net on 3.783 but then I  added 10m to my fan dipole and things changed - may have been something else.

Thanks for your advice, will let you know the final analysis. PS it sure is sensitive to the variable cap adjustment.

[Ve3dgy]

So I’ve gone across the bands, getting SWR 2:1 or lower. Interesting data point: none of the bands transmit over 50W (reflected <5w), according to the tuner, the radio meter echos that. So if reflected is only 5w and output is set to 100W, what’s with 50W output?

[VA3KOT John]

So I’ve gone across the bands, getting SWR 2:1 or lower. Interesting data point: none of the bands transmit over 50W (reflected <5w), according to the tuner, the radio meter echos that. So if reflected is only 5w and output is set to 100W, what’s with 50W output?

Was that measured with a steady CW carrier?

[Tom VA3TS]

is the transmitter putting out 100 watts into a dummy load?

[Ve3dgy]

Hi Tom, the only dummy here is me. I don’t have a dummy load to use. The YT857 has a small power meter on the front it also indicates 50%. When I put a continuous CW tone through it registers 85W out with almost 0 reflected power. I guess in SSB I’m not supposed to see a peak power right. So then my question becomes how come 85% and not 100-reflected = 95-98w. Maybe I shouldn’t care

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PS to all - I’ve seen one advantage to the doublet, on 3.783 the noise is S7, on my fan dipole it’s S9, interesting. But that’s good, the reason John suggested the doublet was to reduce the noise.

[Tom VA3TS]

It will be tough to come up with solutions without a separate swr/power meter and a dummy load. With radio, like electronics or electrical, the goal is to minimize the variables.  


I've seen these things on KW swap   73

[Ve3ids Don]

I use a doublet here and it's great. As Tom suggested, you need a dummy load to sort things out. I made one from one of these:

https://www.amazon.ca/dp/B07Q2L4V9M/ref=...bGbSEG5JNP

I got mine from eBay much cheaper but this is just an example. I bolted it to a large old heatsink. I have been using it for over a year now without problems. It will allow you to load up and test your rig to know what your rigs normal performance is and then you can deal with the antenna. 

73 Don ve3ids