Discussion in 'Antennas, Feedlines, Towers & Rotors' started by 2M0XTS, Jun 26, 2017.
to compensate for the proximity of other objects etc all a bit trial and error
With insulated wire, in an Inverted Vee configuration, I've found that 32ft 6ins usually makes it resonant in the middle of the band. (7.1 MHz)
But as has been said, you will need to trim it to suit your actual QTH)
You can use an unbalanced tuner with a balanced antenna by adding a balun at the feedpoint. However, you are relying on the balun to provide the entire voltage difference to balance the antenna so that the voltage on the two halves of the dipole are the opposite of each other. This means there will always be a very large RF voltage drop across the balun when its transmitting. If the choking impedance of the balun is not high enough, there is still some common-mode current, which multiplied by the large voltage drop across the balun, could result in the balun dissipating significant amounts of the power. Therefore if this approach is used to balance the antenna from an unbalanced tuner, the balun must be designed to have an extremely high choking impedance, in the many thousands of ohms. This is especially a problem at the lower bands, and often requires 3 or 4 stacked FT240-43 or FT240-31 cores with 10-12 winds of coax around the core stack.
Because a large impedance is required across the balun, other ways of coupling between the antenna and the coax must be minimized so that the coupling is not "bypassing" the balun. For example, the coax should be perpendicular to the dipole if possible. Secondly, the balun is a big inductor with a high voltage across it. Placing the balun in an insulating box away from as many objects as possible that can capacitively couple to the high voltage at the two ends of the balun will also reduce leaking of RF and bypassing the balun.
On the other hands, balanced tuners only can be perfectly balanced if the entire load is is also balanced, for example, using balanced open-wire line and a dipole that doesn't have any objects coupling to either side of the dipole. So both solutions have their problems.
@NH7RO, @G0JUR, @G3YRO - ah, cool - I wasn't sure if you were compensating for something in particular or if I'd just got the formula wrong (to be honest, I was just making each wire 1/4-λ and trimming it for impedance matching - to give a total length of 1/2-λ).
In any case, we're still doing an post hoc compensation (that is, compensating for unknown variables after the antenna is up, rather than the opposite - ante hoc design, which would involve compensating for known variables before the design is actually built) - just I was wondering if I was forgetting something when I was trying to get the antenna design into the ballpark!
@KW4TI: Thank you very much! Quite a thorough description you've provided me with there!
If the antenna is resonant you shouldn't need a Tuner at all !
And in your situation I wouldn't bother with a balun either (only with unfortunate coax lengths can you sometimes get slight changes in the radiation pattern)
Let us know how you get on!
Add a second set of whiskers for 20m, so that you don't need to retune both bands later.
I had one of those weekends - you know, the sort of weekend where everything that can go wrong goes wrong. Usually, but not always, at the worst possible moment!
Friday: finished stringing it up - but the SWR was through the roof (the impedance was close to 0) so I scratched my head for a bit, adjusted the lengths of the wires, and then gave up for the evening.
Saturday: took it down, re-made all the connections, and put it back up - SWR was down at 1.8 for 7.15MHz, seemed to be resonant at 8.1Mhz - good enough for some testing! It was a nice day - a little breezy, but sunny, so I said to myself "ah, I'll bring the radio outside for a change" - so I went in, unhooked everything, and went outside... plonked the radio down on the table and... "HI-SWR" warning pops up. Huh. So I get the analyser back out, and the SWR is through the roof again. Weird... and then it started raining, so I gave up for the day and pottered around town with my handheld - decked out in waterproofs, such a fun way to spend a saturday afternoon!
Sunday: took it down again, re-made all the connections again, strung it back up again, and the SWR was back where it should have been - so I started adjusting the length of the house-leg and... *ping* - I end up with ~5 meters of wire in my hand: it snapped roughly half way along it's length. So I went into the backyard; lo' and behold, the feedpoint is hanging off the telephone cables - nice. So I took it all down again and measured out the cables and whatnot - put it all back up and... *klunk* - the feedline falls out the SO239 connector (okay, no escaping it - that one was definitely my fault ) - so I took it down for the umpteenth time, strung it back up - and....
IT WORKS! Currently resonant at 8.08MHz, with an SWR of 1.3, moving to 7.15MHz the SWR is up to 1.8 - still needs tuning (I was too frustrated to be bothered actually tuning it on Sunday - that's tonight's job!) - but it certainly seems to work!
What a rough ride that was :S
I'll post up a rough schematic of it soon - along with semi-regular updates on the other HF designs I'm going to try out (offset-L, delta loop, etc)
I'll also post up the antenna specs once I figure out how to use my SARK100 with my PC (I tried to figure it out during the week, but I hit a handful of driver problems and never got it talking...)
@OH5NXO - sorry, might you mind defining "whiskers" for me in this context? Are we just talking about a second dipole coming off the same feedline (a bit like a fan dipole)?
Ps - thanks for all the help folk! I know I've said it before, but it's very much appreciated!
Just that. Not that the 20m legs should affect 40m a lot, or if you need/want them at all, but ...
Don't mean to be a party pooper but with so many mishaps in one spell I suspect that your soldered connections might not be up to par---and that it won't be long before the SWR is wildly intermittent with the first wind or storm or bird on the wire to jiggle it up. Hopefully I'm wrong about this but if not, you'll know where to look... GL in any case; I hope you succeed in your endeavors like the rest of us.
Great story to read over breakfast, too; we've all been there before multiple times (part of the inescapable "adventure" inherent in our hobby).
Haha - lol, no party pooping involved
I suspect the wire was damaged roughly were it snapped. The legs of the dipole are crimped and soldered onto a d-lug. Those are connected to the feedpoint enclosure using wing-nuts and washers (duly plastered in electrical tape ) - finally, the inside of those nuts are connected to the SO239 directlly (and once I get round to it, via a 1:1 balun) - so if there's a solder issue it's inside that enclosure - it all looks okay just now though, so hopefully it'll be fine for a while!
The cable that snapped seems to be alright (apart from where it snapped) - I might see if I can re-purpose it for another antenna, since the crimps are still attached, and hopefully the wire is intact along what's left.
Tbh, I suspect it was just the cheap ebay-special wire I used - braided steel really shouldn't snap cleanly like it did - it should fray long before it breaks through The current cable that's up is audio wire (1.5mm2 copper in a clear sheathe) - we'll see how long it lasts!
Anyway, I think I'm going to take advantage of the last of the sunlight to do do some DX-ing - might even try out some CW while I'm at it (when it comes to morse, I'm an expert lid )