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Trap-style Inverted-V Antenna for 3.5, 7, 14, 21 & 28 MHz

 

Trap-style Inverted-V Antenna
for 3.5, 7, 14, 21 & 28 MHz

i.e. 10 to 80 metres


March 2010

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Have you ever thought of setting up a for a low-key DxPedition on some exotic island as a side-effect during a holiday ?
Or a portable HF station for an activity like our John Moyle Field Day here in Australia - held in March each year ??
No, then check out a few of the other pages on this web site for ideas about going "portable" (use the menu at the top of page with references to antennas or Field Days).


I have been operating in the various Australian VHF/UHF field days over the last year or so but I only operate in one field day that has a HF component - the John Moyle. Last year I used a trap vertical with a trapped radial system but this year I wanted to be a little better prepared for the HF side - so I set myself the task of making up a completely new antenna - and with space considerations in mind, decided that it would be a trapped inverted-V style for the 5 non-WARC bands. To achieve that, I set about making up some new antenna traps, gathered a spool of wire, crimp / solder lugs and tools to start making this new antenna - with the idea that I wanted it light-weight, easily packed, small but still frequency flexible.


The John Moyle Field Day activity last year was mainly on 80 metres (3.5MHz), 40 metres (7MHz) and 20 metres (14MHz) but as the sunspot cycle advances through Cycle 24, the other higher bands will come to life more so why limit the antenna to just these 3 lower bands, and if I go on a DxPedition/holiday then I will probably want to work 15 and 10 metres too. The other aspect that needed attention was that I did not want to have to run an antenna tuner - the 'natural SWR' had to be low enough that a standard HF transceiver could be used directly with it. My antenna tuner is a manual MFJ-941 series and to change bands then you need to change the switch and capacitor settings on it too - if you remember - or you start to wonder if the antenna is "crook" on that next band..


Construction-wise, there are other pages on this site that deal with how to make and tune coaxial traps for antennas ( see the menu above).


I was thinking ahead last year when I bought a 7 metre "squid pole" to support the centre of a multiband inverted-V for field day use. How I mounted the bottom of the squid pole can be elsewhere on this web site. That may not be the final methodology that is used for field days but at least it gives one idea as to how it 'can be done'. Lower down this page, I suggest some concrete bucket weights that I made up to make the testing easier - and plan to use them in the field day environment too,


The antenna details later on this page are specifically what I ended up with on the completion of the project - wire lengths, SWR values etc.. I am not going to provide a step-by-step on how to make one up simply because most individuals will do it "their way" anyway..


One question you may well ask is "Why not a full-size inverted-V" ?
The thing about a full-size inverted-V is that it is in essence a half-wave dipole mounted high in the middle and then low at each end. As such it has many of the frequency response characteristics of a dipole : a fundamental resonance (eg 7MHz) and operation at odd harmonics ( x3=21, x5=35, x7=49 MHz....). Of these, only one harmonic may be useful for amateurs - ie 21MHz - and that depends on where in the 7 MHz band the basic antenna is tuned.... If it is 7.080 then the 3rd harmonic is 21.240 & is ok but if at 7.2 then the 3rd harmonic is at 21.6 MHz - and is above the amateur band.


If you want to effectively parallel extra half wave sections at the centre to become a ray-dipole style antenna, the issues that arise is that it becomes hard to tune and each leg must be separated by about 30 degrees from the other. That makes a tri-band inverted V needing 6 anchor points, 2 for each leg - and makes it a physical nightmare - and quite large. If you re-locate such a monstrosity (eg for a field day) then you will probably need to re-tune it again because the centre and end heights plus ground conductivity may be different. The inverted-V has a more rounded polar pattern than that of the figure-8 of the conventional dipole so if you can't orient the dipole on your property for the best DX directions because of physical yard dimensions, making it into a V-shape can partially resolve that for you.


The one thing that many amateurs have no real control over is the centre-point height, this being limited by physical limitations eg mounting pipe lengths, so the typical inverted-V is not mounted with optimum apex angles to obtain true omnidirectional patterns. Note that the polar pattern and tuning changes with height above ground too and that optimum at 7 MHz is a much greater height than optimum at 28 MHz. If in doubt, mount the V so that it's axis is perpendicular (90 degrees) to the desired maximum lobe direction (eg mounting the V along NE/SW gives maximum lobes at NW/SE, i.e. favouring Europe from Australia via either long or short paths).


The trapped inverted-V has a few advantages and a few shortcomings over the mutiple-V configuration :

(1) It is about 50% of the length end-to-end. For example, a half wave dipole (before shaping into a V) is about 40 metres long at 3.6MHz. The trapped-V described on this page is about 21.5 metres long.

(2) There is only one leg to provide an end-mounting for on each side with the trapped-V, a 5 band inverted-V might have as many as 5 legs each side so is more difficult to install and tune.

(3) The trapped-V has narrower bandwidth on each band than a conventional V. This is not a major issue if you only operate in one section of the band and tune the trapped-V for that segment.

(4) The trapped-V is less efficient than a full-sized antenna - but then again you can't have everything if you want a multi-band V.

(5) Finally, the trapped-V is smaller and lighter than the full sized V - just taking into account the copper weight alone. That makes it easier to set up by yourself, transport away on a holiday,......


If the advantages outweigh the disadvantages for you, read on.....

 

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I have referenced the following web page and associated software elsewhere on this web site but if you haven't yet seen it, check out "the Coaxial Trap Design software by Tony Field VE6YP - { & download: coaxtrap.zip & read the HELP ! }.
I have used this trap calculator successfully a few times before and can heartily recommend it as a starting point, noting that the labels for metric versus imperial don't indicate properly if you actually re-start it in metric mode.


Construction tip : ALWAYS allow about 5-10% extra coax above the calculator's displayed length so that the relevant trap is always low in frequency. You can take a short length off the coax to raise the frequency easily - but joining more coax on to lower the frequency is a different matter !


My 8 traps (4 each side/leg) were based on 20mm OD white plumbing pipe/conduit for 28 and 21 MHz and 48mm for the 14 and 7 MHz traps, 2 for each band - or one for each side of the 'V'. All PVC tubes used were cut to 75mm lengths, regardless of diameter.
I used the best part of 10 metres of Belden RG174U miniature 50 ohm coax in making up the 8 traps. There is a little left from the 10 metres I started with - but not much.
Please note that my field day HF transceiver only outputs 100W PEP so there is no need for heavy duty componentry - like RG58 coax cable and heavy gauge antenna wire. A trap using RG174 should survive at 400W PEP anyway but the kilowatt crowd would burn it up in an instant.


As with my earlier traps, the formers were slotted towards one end and the trap wire ends were secured onto solder lugs that were fixed via holes in the end of the PVC formers with 4mm screws (25 mm long), flat washers and nuts.
The final as-constructed values against those calculated are as follows :

Freq Design values from VE6YP calculator Final Turns Used
28.4 MHz 5.93T, 20mm dia, 1.51cm long, 44.54cm ** coax TBA
21.2 MHz 7.63T, 20mm dia, 1.95cm long, 56.57cm ** coax TBA
14.2 MHz 4.7T, 48mm dia, 1.2cm long, 77.34cm ** coax TBA
7.1 MHz 8.09T, 48mm dia, 2.05cm long, 130.93cm ** coax TBA

** ALWAYS ALLOW EXTRA COAX - SEE CONSTRUCTION TIP ABOVE


The actual trap tuning process is documented elsewhere on this web site


These coax traps do not need to be overly fine tuned so that they are exactly on the same frequency - in most cases 'near-enough' is ok ( i.e. within 10-20KHz of each other ). They are low-ish in Q and provided they are close to the desired frequency, the actual resonant frequency is not super-critical.


The actual resonant frequencies seen in the finished antenna are mainly the result of the wire lengths and the inductive reactances produced by each successive trap - rather than the actual trap frequencies. The purpose of the trap is to present a high impedance at the resonant frequency ( in effect an automatic segment switch) but just appear as a series inductance to the next lower frequency segment along the wire.

Trap coil former set
The trap formers ready to start manufacture.

Completed trap set
The set of 8 traps ready to make up the 'V'.
Red (at LHS) = 7.1 MHz, blue = 14.2 MHz & the pairs of white are 21.2 and 28.4 MHz (RHS) respectively.

The following images are of the earlier series 7MHz trap (after being taped up) but the new traps were made in exactly the same manner, even if a different size former.... (mouse over the images for larger view)
7MHz trap
7Mhz trap showing slot
7Mhz trap with adjustable tail
This image shows the 38mm PVC former after being taped up. A close look on the larger image will reveal that the turns are spread apart towards the closest end. Note the use of 4mm screws, nuts, etc and crimp lugs (soldered too) for the incoming, outgoing and tail wires. The inside view reveals the slot used to make the tuning easier -far easier to expand the "coil" as against a small fixed hole.. And by making the coax around 6 - 10% longer, you will almost always have to "spread the turns" ! The overall image shows the "40 metre" tail attached. In practice, it simply 'droops' down from the trap.


The traps were made and I was then ready to do the wire segments in between. The basic 1st segment ( A ) is a 1/4 wave at 28 MHz so that was easy - set it at around 2.5m - as "close enough' - and work from there. One of the things I did like about Diamond's W-series trap dipole design was that the final tuning was done with droopy "tails" that were usually trimmed with cutters to set the final resonance point - rather than adjusting the actual spacing lengths between the relevant traps. Oh, so much easier - and because my mounting had to be just that bit more flexible because the environment would change from one field day outing to the next, I needed to be able to quickly do a "field tune" - only as necessary of course. Instead of cutting the tails to the exact frequencies, I simply wound back the tails along themselves to effectively shorten the length - and thus set the resonant frequency - but next time, I could alter the wind-back to move the frequency rather than adding a new tail to each side and then go through the entire tuning process from scratch for that band.


I made up a W2DU-style 1:1 balun in a 20mm white PVC plumbing pipe/conduit tube with a BNC female coax connector on the bottom, screw connections at the top, and hung it from the squidpole but it's weight was simply too great and the pole was nowhere near vertical. I didn't even get a chance to see how well it performed because of the weight issue and have put it to one side pending experimenting with other HF antennas at some future time. For any other centre mounting method, it would have been fine.


In its place is another shorter piece of 20mm conduit with similar connections / construction and also with a BNC female coax connector on the bottom - but without the ferrites as the "sleeve". Yes, that means there is no balun in place - but this antenna is for short-term field day use, typically in a remote location where TV, radio or audio interference is not going to happen.


I "stole " the graphic below from one of my other web pages so that I didn't have to redraw it because the inverted-V antenna is basically 2 of these identical sides fed from a central feed point (via wire length A), usually via a 1:1 balun of some sort. Delete the reference to the word 'radial' in the graphic when you look at it. One difference in practice with this antenna is that there is also a 14 MHz adjustment 'tail' connected at the LHS of the 14MHz trap, not shown on the drawing below.

Trap arrangement detail


I used insulated 14x0.020 insulated hookup wire - maybe too light for a permanent installation - but this is a field day antenna where actual weight, size and ease of untangling and putting it up quickly is of the highest importance, not to mention the loading on the squid pole. Besides that, I had a couple of 100 metre rolls with a black insulation in the shack - and it only goes up in the air for about 8 hours at a time during a field day !


The wire segments were terminated onto crimp lugs (subsequently soldered as I detest crimp-alone connections) which simply slipped over the 4mm trap termination screws and were retained by an extra 4mm nut & lock washer. Where tails are required (except the 80m one), the tail wires are also terminated on a solder lug and again held in place that same 4mm nut. If you trim the tail too far, simply remove it by unscrewing the nut and replace it with a new tail with a longer wire. No length adjustments between the traps are necessary during tuning - I have done all of these length adjustments already and the table below contains these final values.


Recommended : initial tail lengths should be about 300mm, initially trimmed back with sidecutters during the tuning process until the resonant frequency is around the bottom edge of each band respectively (eg 3500, 7000, 14000, 21000 { & 28000 where a tail is fitted } KHz). After that, just twist/wind it back on itself back up towards the termination point to provide a length cancellation effect and thus shift the resonance up to the portion of the band you want to operate in ..


Remember to tune "down in frequency" ie. adjust 28 MHz first, next 21, next 14, next 7 and finally 3.5 MHz.
In this design using tuning tails, it is not as critical to follow the "tune down" process as it would be on a design "without tails", as the lengths between the traps should not change in this style - only the tail lengths !


Final spacing values turned out as tabled, all using insulated 14x0.020 multi-strand copper wire (if uninsulated used, add about 2% to each length and adjust length on test ) :

Wire position Drawing Dimension Length Adjustment tail, connected to "inner end" of the outer trap
"Balun" screw terminal to the 28 MHz trap A 2.300 m 28 MHz tail - None required - resonates at 28.5 without one ( probably this section could have been shortened by 10cm or so and a 'tail' added to make setting up easier )
28 MHz trap to 21 MHz trap B 15 cm 21 MHz tail - 85 mm tail (adjusted for desired centre freq on 21 MHz eg 21.200 ) { 21 MHz wire length is effectively A + 28Trap + B + 21 tail length }
21 MHz trap to 14 MHz trap C 49.5 cm 14 MHz tail - 160mm tail { 14 MHz wire length is effectively A + 28Trap + B + 21Trap + C + 14 tail length }
14 MHz trap to 7 MHz trap D 2.650 m 7 MHz tail - 155 mm tail (adjusted for desired centre freq on 7 MHz eg 7.090 ) { 7 MHz wire length is effectively A + 28Trap + B + 21Trap + C + 14Trap+D + 14 tail length }
7 MHz trap to end E 5.00 m 3.5 MHz tail - 250mm tail, drooping from egg insulator termination (adjusted for desired centre freq on 3.5 MHz eg 3.590 )


Note : Below their individual resonant frequencies, the 'traps' appear inductive thus shortening (by inductively loading) any subsequent 1/4 wave wire length.
And, yes, these lengths are different to those tabled on the Field Day HF antennas page about a tuned trap radial system.


Mouse-over for a larger view of the image....

My W2DU-style balun - the 20mm conduit contains a series of cylindrical ferrites

The terminations inside the top of the tube : soldered crimp lugs + 4mm NP screws/nuts/washers.

Physical strength for the coax tail : a couple of nylon zip ties through holes in the bottom end of the tube. Heatshrink was also put around the female BNC connector body and zip-tied back to the coax sheath so that the weight would be distributed - rather than just on the connector termination.

This is a view of the "no-ferrites" feed tube attached to the top of a squidpole and with the inverted V sides and coax feeder attached.

This shows the 28 MHz trap (top), 21 MHz (centre) and the 14MHz (bottom) traps on one side of the V. Twisted tails are fitted to the 21 and 14 MHz traps

This is the 7MHz trap on the same side of the V. Note the twisted tail.

View up to the top of the inverted V when mounted on the squid pole. And, no, it won't stay up straight with the antenna weight loading it. Three traps on one side are easily visible on the expanded image.

This is the finished inverted-V all packed up after tuning.The longer central PVC tube is the "ferrite-less" centre feed, BNC female connector at the bottom.

This is my test feeder - about 15 metres of RG58-sized low-loss 50 ohm foam dielectric coax, terminated in BNC male connectors each end.

The 7MHz trap after a foam filler was used and after being cut off flat with a long-bladed knife - after the 24 hour set time.

The 'bubble hole' effect is right through the filler so has little mass - but it will protect the internal connections from rough handling.

My end support poles are made from 40mm & 45mm white conduit, telescoped to provide an end height of 2.5 metres.

The tail visible is the 80 metre adjustment, again twisted back on itself.

The top end of the end-tube has 2 diagonal holes - one for the incoming inverted-V support wire/rope and the other side for the connection of a guying rope (if required)

The concrete bases (info at right) are not really heavy enough to hold the squidpole up if windy so a 3-direction guying plate with 3 easily-adjustable nylon ropes is used to prevent it from toppling.

One of 3 concrete base supports for the squidpole & end tubes - about 13KG of concrete in a plastic bucket. The metal loop at the RHS is actually an old automotive muffler clamp with the enclosing section and nuts immersed into the concrete while still wet. One was fitted to each bucket.

More info about these in the text below !!!

The squid pole with its 40mm conduit sleeve fits well into one of the base supports

Slightly more side view. The tube set into the concrete is 275mm long, 50mm diameter.

The collapsed squid pole sits easily in one of the bases. The white tube at RHS is one of the 2.5m end support conduits

Better view of guying plate with the squid pole mostly collapsed.

 


Once it was "tuned" by tail adjustment to the desired bands, it was time to go through and do SWR checks again and see how good/bad it was - in an overall sense. The tests were done using an IC-718 HF transceiver with the lowest possible RF output power - only enough to drive the SWR meter to the CAL position.

Band Freq of actual SWR min (KHz), SWR 2:1 SWR point - Low, High (KHz) 3:1 SWR point - Low, High (KHz)
3.5 3590, 1.25:1 3550, 3630 { 80KHz } 3525, 3660 { 135 KHz }
7.0 7120 *, 1.4:1 7075, 7160 { 85 KHz } 7050, 7190 { 140 KHz }
14.0 14120, 1.6:1 14070, 14175 { 105 KHz} 14020, 14240 { 220 KHz }
21.0 21250, 1.5:1 21120, 21395 { 275 KHz } < 21000, > 21450 { > 450 KHz }
28.0 28280, 1.4:1 < 28000, 28750 { > 750 KHz } < 28000, > 29000 { > 1 MHz }

* 7120 KHz is a bit high in the 40 metre band for normal operating but, for the purposes of testing the antenna, it allowed the lower limit to be explored a bit more - i.e. down to band edge.


I prefer an SWR of under 1.5:1 and the only band that this doesn't occur with resonance is 14MHz. Then again, the tests were done with the antenna partially over a sheet metal patio roof so are likely to change somewhat in a normal FD installation. Given the normal feed impedance of a 1/2 wave dipole is around 70 ohms at resonance, the above numbers seem reasonable with a 50 ohm feeder ( eg 70 ohms = 1.4:1 SWR).


Something that must be mentioned at this point is that these are 'bare' SWR figures - no antenna tuner was involved. If you are using a transceiver with an internal or external tuner facility then the effective SWR at the transmitter output will be 1:1 (after matching) BUT the efficiency of the antenna away from it's resonance may be quite poor. The further away in frequency then the poorer the performance.


The antenna creation process took about 1/2 a day in the workshop - that included building and testing the 8 traps, terminating the various length trap interconnection wires onto the crimp/solder lugs, plus making up 8 x 300mm 'tails'..


The outside work, the actual interconnection of the traps, adjusting lengths and the whole tuning process and doing an SWR run across each band, took the best part of another day (interruptions included)..


As expected, the narrow bandwidths on 3.5 and 7 MHz are caused by (1) the actual percentage of a whole half-wave dipole that the active section uses (eg about 11.5m of 21.1 m = 54% at 7 MHz, 21.5 m of 41.6m = 51% at 3.6 MHz, or in other words, these lower segments are heavily inductively-loaded) (2) the thin wire gauge used (3) the proximity of the ends to the ground (4) and, to a minor extent, the Q of the two 7MHz traps on each side of the antenna itself. Even so, provided I centre my 80/40 field day operations around the respective band resonant frequencies, this antenna arrangement should be quite a reasonable option.


One of the last things I did after checking out the operation on all 5 bands was to fill the PVC trap formers with an expanding poly-foam filler (available in spray-style can, used for moulding around and shipping goods & filling gaps behind wall panels) to provide some additional robustness and weather protection, rather than trying to find end caps to fit... The filling was only partially successful as I did not have the can completely inverted (as per recommendation on the label) so some chambers were more air than they should have been ! Oh well, I'll know better next time. Even so, the terminations inside are now somewhat protected from physical damage through handling, packing and falling heavily onto the ground when letting the antenna down and with the poly filling being mostly air, there is no appreciable extra weight involved..


While I didn't measure the weight before the filler was added into the traps, the total weight of each side is now 350 grams for the 4 traps plus wire plus the plastic egg insulator. The centre termination is just 50 grams. That makes the entire inverted-V assembly just 750 grams in total.. If you added a 20 metre spool of 3mm nylon rope then you are still under 800 grams so that makes this antenna really suitable for taking along on that exotic holiday. Add some RG58 foam-style coax (lighter than poly inner style and lower loss), a small HF transceiver and 'quiet' switch-mode power supply and it could all come true. By the way, once the filler sets (leave it for 18 - 24 hours) it feels a bit like a heavy duty semi-plastic porous plug - that means it is soft enough that you can use a standard snap-off utility knife (with the blade fully extended) to trim off the end of trap former "bubble-out".


The centre mounting, particularly for testing at home, would be a lot easier with a centre weight that would just take the cylindrical base section of the squidpole & some end weights - something that would make the whole inverted-V / squidpole mounting easier in an actual field day environment too. One idea is to :
(1) Create three concrete weights (in 10L plastic buckets) that have suitably-sized PVC conduit sleeves set vertically into the middle, complete with an eyehook of some kind for fitting a lifting handle or rope.

(2) Set up two 2.4m plumbing pipe/conduits ( to comply with ACMA EMR rules / antenna to person/animal spacing ) that fit into sleeves in the concrete weights, with a tie-off / termination hole in the top - for supporting the ends of the inverted-V.


I thought that for the cost involved in trying out this arrangement with a few plastic buckets plus some concrete - well, why not !


The weights were made up with 2 x 20KG bags of concrete pre-mix - "postcrete" - (about $6.50 each) , 3 $0.90 plastic buckets and some pre-used 50mm plumbing pipe/ conduit for the sleeves. That makes the three weights at around 12-13-14KG each ( about 2/3 bag each ). The concrete needs to set for 24 hours before use... & 10mm drain holes were later drilled into the bottoms of the buckets in line with the tubes after the concrete had set, just in case it rained and needed to drain away.


In practice the end weights are almost sufficient ( gusty winds and uneven ground excepted) with a 2.5 metre plumbing pipe/conduit endpole but the squidpole when loaded with the inverted-V has a tendency to move around. If it got really windy then it would topple. The work-around was to fit a 3-direction guying plate with 3 easily adjustable nylon ropes and attach them to tent pegs. I also added two extra ropes so that the end poles could be guyed out as well - one each in line with the legs of the V. The ropes are intentionally yellow for good visibility - I don't want to trip over them ( or have anyone else do so ) during a field day setup !


Photos above......

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An interesting effect was noted during the testing of the inverted-V : The frequency of the SWR minima changed downward over a short period of time. Weather - gusty winds. My assumption is that the interconnecting wire lengths have in fact stretched with the wind gusts - to the extent that the 28 MHz frequency moved down by 30 KHz. While this may not seem to be a major frequency change, it is an indicator of why the resonance dips on wire antennas actually move downward over time. I was "lucky" enough to measure the frequency as soon as the antenna was erected so got the un-stretched frequency but then I went through and did SWR tests on the other bands so it was probably about 1/2 hour later that I went back to 28MHz and, voila, it had moved down 30 KHz. When I went and rechecked 7MHz, it too had moved down ( actual amount not noted ). During the interim, the antenna has not been physically adjusted, moved or anything else so the only thing that accounts for it is wire-streeeeeetch.


The moral of this story is to either (1) pre-stretch all antenna wire before measuring/using it (this being the reason that professional long-wire/dipole antenna installations use hard-drawn copper wire - it doesn't stretch) - or - (2) go back a week or two later after it has been up in the air continuously and reset the lengths to shift the frequencies back where they should be.

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Of course there is nothing to stop you adding extra sections for 10, 18 & 24 MHz as second legs - just as I did with my permanently mounted W8010 - and as described on this  web site - to make it into an all-band inverted-V.

 


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