VK2ACY - G5RV coupler
My G5RV Dedicated Coupling Unit for 80 / 40 / 20M
To my knowledge (despite the reams and terrabytes already expended on discussion / argument / tutorial) re. the G5RV, nobody has ever designed a dedicated coupling unit just for this often misunderstood antenna. After exhaustively confirming absolutely NO difference between my G5RV and (full-sized half wave centre and end fed) dipoles on 80 & 40M at my QTH, the decision to retain my G5RV as primary antenna for these bands prompted my designing such a unit with fixed settings, providing replicable swr readings ( without even the need for me to check each time used ). My prototype used all secondhand parts, namely:
One HF toroidal balun core, 6 metres of 0.63 or 0.71mm enamelled winding wire, 8 inches of 3/4inch diameter wooden dowel rod, 12 inches of hookup wire, 6 inches of thin tinned wire, three small alligator clips – white, black and green, 2 only 680pf 250V ceramic / styroseal or similar capacitors, one SO-239 socket, two insulated banana terminal / binding posts, 4 small brad tacking nails, 2 small self tapping screws, and an old MacKintosh’s Chocolates & Toffees candy tin with a handy swing lid. ( 8 x 5 x 2 _ inches W x D x H )
The attached pictures show what I did and this is the order in which I did it:
a.Mounted the SO-239 socket in the middle of the rear panel of the “toffee tin” and then mounted each of the insulated banana terminal / binding posts either side of it.
b.Cut two 2 ft pieces of the enameled winding wire, and wound 10 turns of each parallel to each other (ie: side by side) tightly around the balun core, ensuring each of the pairs of turns tightly follows the outside / inside surface of the “doughnut”. I spaced the windings about 2.5mm apart, ensuring that the 10 (pairs of) turns covered about three quarters of the doughnut’s entire circumference.
c.Left about 2 inches of winding wire at each end of the windings before cutting off the excess. Identified the “start” and “end” of one winding (primary winding), trimmed these close to the required length, and (using a sharp knife) scraped enough enamel off the wire to permit tinning with solder and soldering directly to the SO-239 connections (the “start” of the primary winding soldered to the centre pin, and the “end” of the same winding soldered to the chassis / shield lug.)
d)Similarly scraped and solder-tinned the “start” of the other winding (secondary winding) leaving it ready for soldering later as required. Also scraped and solder-tinned the “end” of the secondary winding leaving only enough wire so that folded once over on itself it will form a 0.5cm “tab” which a crocodile clip could manage to grip firmly onto. This “end of winding” tab will form one balun connection which is selected by moving the appropriate “crocodile clip” when using the antenna on the 40 & 20M bands. I labeled this connection to the balun: “Full Winding Tap for 40 & 20M.”
e)Counting four whole turns back from this 0.5cm “end of winding” tab, a small area of enamel is scraped off the top of the same wire which forms the secondary winding to enable solder-tinning and a small 1 cm tab of wire to be also soldered on at that point. This 1cm tab will similarly form another balun connection which is selected by moving the appropriate “crocodile clip” when using the antenna on the 80M band. I labeled this connection to the balun: “Half Winding Tap for 80M.”
f)Turning my attention to winding the required 7 uH tapped inductors, I hammered two nails into the dowel rod, each half an inch either side of the exact middle of it’s 8inch length, leaving only about 3mm or so of the top of each nail exposed. I then hammered one of the remaining nails exactly two and a half inches further out towards the left-hand end of the dowel rod, from the left-hand nail. Then I similarly hammered the last remaining nail exactly two and a half inches further out towards the right-hand end of the dowel rod, from the right-hand nail. I found that these nails formed convenient anchoring posts for the enameled wire about to be wound onto the dowel rod.
g)I then cut a 2metre length of enamelled wire and scraped an inch or so of enamel off one end to permit anchoring by wrapping twice around the left-hand nail closest to centre of the dowel rod. After tinning / soldering it in place, I then proceeded to tightly wind the wire around the dowel at a pitch of 15 turns per inch, for a total of 37 turns, arriving just near the outer left-hand anchoring nail. I temporarily held the end of the winding in place with tape while I cut the wire (leaving one inch for scraping / tinning, anchor – wrapping and soldering around the nail itself.)
h)Once this was done I proceeded to repeat the exercise for the inductor to be wound on the right-hand side of the dowel rod, winding carefully and making sure that the direction of the winding was the same (ie: not counter-clockwise with respect to) the first winding I had made. These two identical inductors would eventually be “crocodile clipped” in series with each leg of my G5RV ‘s feedline, depending on the frequency band in use.
i)Counting from the very centre of the dowel rod (where the “starts” of each inductor’s windings are located) I carefully marked with texta at the 8, 17, 18, 19 & 20 turn points on each inductor, and proceeded to scrape a small area of enamel off the wire at each point to permit solder-tinning and attaching a small connection “tab” (using very short pieces of the thin tinned wire doubled over) to which a crocodile clip could grip firmly onto.
j)At this point I breathed a sigh of relief because I knew that the unit was JUST ABOUT FINISHED !!!!! and proceeded to mount the dowel rod on which I wound the inductors by screwing the two small self-tapping screws through the sides of the tin case and firmly into the ends of the dowel rod to prevent it spinning or moving. The rod was aligned so that the nail heads and inductor taps were all facing the rear of the unit.
k)I then cut a short 2 inch piece of insulated hookup wire and soldered one end to the inside of the insulated banana terminal / binding post on the left hand side of the rear panel, and the other end to the black crocodile clip. This could then be clipped to the appropriate winding “tap” on the inductor closest to it, for the band in use.
l)Similarly, another short 2 inch piece of insulated hookup wire was soldered to the insulated banana terminal / binding post on the right-hand side of the rear panel, and the green crocodile clip fitted to the far end, eventually allowing connection to the appropriate inductor taps closest to it.
m)ALMOST FINALLY – taking one of the 680pf capacitors, one lead of which was soldered to the left-hand inductor’s “start” of winding connection (at the central anchor nail), the other lead of the capacitor was then soldered to the balun’s secondary winding “start” connection (as mentioned above in item “d.” )
n)The other 680pf capacitor was similarly soldered to the right-hand inductor’s “start” of winding connection (at the central anchor nail). A short one inch length of insulated hookup wire with the white crocodile clip soldered to the far end was then soldered to the capacitor’s other lead. This could then be clipped to the appropriate “Full Winding” or “Half Winding” balun connection required for the frequency in use.
FINALLY – after extensive testing including SWR measurements and on-air comparisons, I was able to sit down and make the following chart to remind me of the “crocodile clip” settings for each band of interest:
80M Band: White crocodile clip connected to “Half Winding” Tap on balun transformer.
Black and green crocodile clips connected to respective inductor taps as per the following centre frequencies: 3.550 Mhz use both taps at 20 turns 3.580 Mhz “ “ “ “ 19 turns 3.610 Mhz “ “ “ “ 18 turns 3.640 Mhz “ “ “ “ 17 turns
(Higher centre frequencies available by tapping appropriately at the ascending rate of 30kHz per turn. Worst case measured SWR was 1.5:1 in between adjacent turn/taps.)
40M Band: White crocodile clip connected to “Full Winding” Tap on balun transformer.
Black and green crocodile clips connected to respective inductor taps at the 8 turns position (covers whole of 40M band)
20M Band: White crocodile clip connected to “Full Winding” Tap on balun transformer.
Black and green crocodile clips NOT connected to inductor taps, but rather: on 20M only these are each attached directly to the capacitor closest to them by clipping onto the capacitor’s lead which is NOT soldered to the inductor rod.
So how exactly does my G5RV Dedicated Coupler actually work ?
Very simply in fact – so much so that I didn’t feel the need to draw a circuit diagram for the completed unit ( I could have built another one in the same amount of time !!! )
The connection from the SO-239 socket is fed directly to the primary winding of the balun transformer. The balun performs the necessary “balancing” in order to run the G5RV as a true balanced doublet, and also performs the necessary step-down impedance transformation when the “Half Winding” Tap is selected during use on 80M. From there, the two balanced balun connections each feed through a coupling capacitor and then an inductor winding on their way to the antenna feedline connections.
As the Q factor of the inductors is a medium-range figure on 80M, the useable bandwidth is limited to about 60kHz, so a number of taps are provided (at 20, 19 , 18 & 17 turns) to reduce the inductance as higher frequencies in the band are used. The use of larger diameter inductors (say 4 inch dia.) would have negated this requirement, but made for a somewhat cumbersome – sized unit to say the least – not to mention greatly increased cost.
On 40M the G5RV is a very broadbanded antenna, so the one set of inductor taps (at 8 turns) more than satisfactorily provides a suitable match without the need to change as higher frequencies are used further up the band.
On 20M the capacitors and inductors are by-passed and the balanced feedline connections simply “clipped” onto the output connections from the balun operating in
1:1 mode. Measured swr right across the whole 20M band was under 1.5 to 1 when operating the G5RV in this true balanced mode (as the reactive coax inductance does not come into play as it did prior to my using this unit)
So there you have it. An afternoon's work and absolutely NO financial outlay, resulted in an SWR of better than 1.4:1 across 80/40/20 and the knowledge that my G5RV is now operated as a true balanced doublet, with consistently up to 1 S point improvement particularly on 80M.
73 from Wayne P. VK2ACY email: email@example.com
P.S: For the record, my G5RV was installed as a flat-top at a height of 25ft and constructed with 51ft of 1.25mm enamelled wire per leg, fed with 28ft of solid 300 ohm TV type ribbon feeder directly to the coupler situated on the ground, and 7.5metres of RG-58 coax. from there to the station’s operating position. Estimated maximum power rating of the coupler built by me is thought to be around 200watts PEP.
Note: Feel free to use or pass on this information as desired (except for commercial purposes or profit) however NO responsibility accepted for any consequences arising. Please email me should you make use of this info as the results of my other homebrew – friendly endeavours will be shared depending upon received feedback.
|Design||Beam * Dipole * Dish or Parabola * DDRR * Log Periodic (LPDA) * Loop * Mobile and portable * Omnidirectional * Panel * Quad and Quagi * Screwdriver * Small tuned loop * Vertical * Yagi-Uda * Wire and random wire antennas|
|Installation||Antenna Tuners * Capacity hats and loading coils * Cavity filters * Coaxial Cable * Feedlines * Rotators * Towers and Masts * VK2ACY - G5RV coupler|
|Theory||Front-to-back ratio * Impedance matching * SWR * Tower design * Vertical Antenna efficiency * Wire comparison tables|