If your going with a vertical dipole, why not investigate the T2LT tuned transmission line trap antenna, can be hung to a tall tree or fitted to a 10m non conductive telescopic mast, its basically a coaxial made dipole equivalent to a Shakespeare big stick but using an rf choke to set length of bottom radiator of the dipole,
or you could also consider it a 1/2 wave version of the Sirio Gainmaster, principle is the same anyway just the Gainmaster was better executed than the Big Stik and was 5/8 wave as opposed to 1/2 wave. Think of the T2LT as what the Big Stick should have been.
I read many things about it online,some true some false, i know some of the guys here have had great results with them compared to either silver rods with lossy coils or A99 with very lossy coil system, some say because it takes use of the skin effect of coax, coax to RF is a 3 conductor cable not 2 when common mode current is present, which it almost definately will be to some extent if your feeding a balanced antenna with an unbalanced feed like coax,you have the obvious centre conductor and the inside skin of the braid for RF to travel on, this antenna takes advantage of the fact RF only penetrates microns into the copper, this means you don't have to fold braid back over the sheath as the RF will naturally flow over bare end of braid and onto braid outer surface when it reaches the point where you bare the sheath and braid and leave inner conductor and dielectric as top radiator.
i read (from a random amateurs page) you don't need to take account of cables velocity factor as it is no longer coaxial after feed point, that is pure bollocks, true it isn't coaxial anymore but all conductors in a dielectric have a velocity factor, won't be .66 or 66% like rg58/rg213/u that antenna and feed are made from, but will be around .97 or 97% - .98 or 98% that is commonly found on all antennas, bare in mind air is also a dielectric so will be about .98 if bare copper in air and about .97 if left in dielectric, even aluminium/aluminum for you US guys, lol, antennas have this trait too, not to mention being deliberately left short to add capacitive reactance so a coil of equal inductance can be added at base to cancel out the capacitive reactance and also provide a 50 ohm tapping point as an end fed 1/2 wave or 5/8 wave has an end impedance in thousands of ohms, base of coil is grounded at o ohms, so in between at some part of coil lies a 50 ohm tapping point. Trying to make a full length 5/8 or 1/2 wave radiator is folly as the dielectric constant of air will always have it slightly shorter anyway.
if you put up a measured 1/2 wave or 1/4 wave or any other wave antenna, bare or coated conductor at full size without figuring in this tiny velocity factor, i guarantee it won't be resonant on the frequency you cut it for as it will be slightly too long and have inductive reactance at that frequency, your mfj 259 or swr meter might not be accurate enough to show it, but it is there, trust me. Anyone that knows anything about antennas will verify that, a high quality vector network analyser will probably show, it but cheap off the shelf stuff may not.
the section above the feedpoint which is central (as in the big stick which has coax running up inside the lower metal tubing) and is not end fed like many people believe because they attach the coax there, should be exactly the same length as the part between rf choke and feedpoint.
its always best to make the antenna slightly longer than required on both the section above rf choke and the bit above where you cut braid and shield away, as it will allow for trimming to resonance, you would trim both parts equally in small increments getting smaller the closer you get to resonance as if you don't you will easily pass by resonance and go from inductive reactance to capacitive reactance, you can use a capacitor in parallel with shield across the choke to make it perfect 50 ohms, but its hardly worth introducing more points that water, your biggest enemy can enter as dipoles have a nominal 72 ohm impedance in free space which is just under 1.5:1 vswr, but its not essential as most radios will tolerate up to 3.0:1 vswr easily, you will also find in the real world that nominal 72 ohm impedance is rarely achieved so its likely your vswr will be slightly less anyway, could be slightly more too, no biggie to modern PA stages.
the RF Choke, ugly balun, call it what you want, dimensions are critical, for rg58/rg213/u about 5 turns 4.25" in diameter offers optimum choking impedance at 27 mhz, other cables will differ slightly depending on what their velocity factor is. trial and error or an mfj 259 or similar with the centre conductor left unconnected and 2 ends of the braid that is to form the choke connected one to meters inner conductor and one to outer/shield connector will give you optimum impedance reading for any type of coax as you change turns and diameters till you find the highest impedance you can, once you find that optimum impedance you will find after the choke, common mode current is so small will be nigh on immeasureable leaving your unbalanced coax almost a perfectly balanced feedline which wont radiate due to cancellation. (i'll wait for the flak on that one, lol).
at the end of the day you'll be left with an antenna that due to being air or non conductive mast mounted will be isolated/decoupled from feedline, have less tvi/rfi/noise pick up and a truer radiation pattern, for the price of a pl259/83-1sp some coax and either a bit of string or other non conductive material to haul it up a tree, or the cost of a 10m fibreglass non conductive mast, which can in turn be mounted to a much longer conductive mast to give height, as the antenna finishes 4m or so from bottom of mast its not going to affect it or choke which can be taped to the non conductive mast or left hanging, take a catapault and some nylon or braided rope to hoist it, attached to a lead weight and fire it over the highest branch you can (or if your as daft as i was in my youth climb the fucking tree) and you have an ideal antenna for portable/discreet homebase operations.
Will work horizontal or inverted v too as its just like any other dipole, but better, all thats left is to weatherproof exposed copper with self amalgamating tape or Starbright electrical insulating paint or both and jobs a good one.
i've told you how to do choke, for antenna dimensions, you'll find 300 divided by centre frequency, (27.205 for US band,27 for 26-28 mhz coverage or for coverage between mid and high band where most dx is done and uk40 and us 40 lie about 27.5 is ideal) you then multiply that by .97 and divide by 4 for each 1/4 wave radiator to get your answer in metres, you want feet multiply that by 3.28 and if you want inches multiply the feet answer by 12, its that simple.
300 / 27.205 x .97 (as you'll most likely leave dielectric on to waterproof) / 4 =
300 / 27.205 = 11.02m x .97 = 10.70m / 4 = 2.67m x 3.28 = 8.77 feet x 12 = 105.25 or 105 and 1/4 inches, figures rounded up or down to 2 decimal places for simplicity.
which is why all those 102" mobile whips you guys buy are so hard to swr without adding a shock absorber coil that has braid running inside it (which shorts the coil) and increases the radiator length by about roughly 3 or 4 inches to roughly exactly what it should be give or take an inch. buy a 2 piece 108 inch whip and you won't need a coil and its tuneable without cutting.
fucking hard putting back when you cut off too much, cutting is a LAST resort, as its more likely a poor ground plane/counterpoise making it hard to swr or poor soldering on coax than the whip.
will work well on any frequency using that formula.
or you could alternatively do my world infamous A99 mod, i fucking love that one, (one for you Carmen
, i know how much you love it too lmao) :
