Antenna Amplifiers 4 element yagi PA70-4-1.5B for 4m ham radio band.
Antennas Amplifiers’ 4 element yagi for the 4m / 70MHz ham radio band makes a good overall impression. It took me less than two hours to assemble and mount it on my telescoping tower. Soon after, I had my first amateur radio connections on 70 MHz in the log.
Figure 1: Antennas Amplifiers PA70-4-1.5B 4 element yagi for 4m ham radio band on my telescoping tower.
Shipping and Packaging
Ordered from WiMO[2], the antenna shipped in four business days. Packaging is 1.49 m long, just so it fits the 1.47 m boom. Element rods came grouped by size in cling wrap. All hands-on in sympathetic small business look and feel.
Figure 2: packaging of the antenna with parts in cling wrap.
Initially I was interested in the 3 element version because I was concerned that the 4 element might be too hard to handle on my balcony. Since it was sold out in Germany, I even considered ordering it directly from Goran YU1CF[1]. However, shipping from Serbia is said to be slow and costs 40€. In the end, I decided to do a bit heavier lifting and went for the 4 element variant.
Assembly
The enclosed English manual does not cover the PA70 types specifically. It just contains some technical details for a variety of antenna products. For me this wasn’t an issue as all parts are clearly labeled, making the assembly fairly self-explanatory.
Figure 3: Assembly detail mounting elements to the boom.
Antenna elements numbered 1 – 4 with letters A and B indicating sides are all matched by labels sticking to the aluminum boom.
Figure 4: Antenna element assembly with labels and drill holes for self-tapping screws.
Furthermore, labels E1 through E8 mark where smaller and larger diameter pipes that make up the elements fit together. Push in the smaller pipes until both drill holes for self-tapping screws overlap. Afterwards put in the screws to secure the elements. One little point of critique: I might have wished for slightly smaller drill holes to give the screws a better grip.
Figure 5: Fixing the balun to the active elements of the antenna.
My next complaint is that I could not find a wrench to tighten the screws that fix the balun connectors to the active elements. Even with some more space around the screws the cable roll still blocks the access. This was the only part of the assembly that I found a bit fiddly.
Operating the Antennas Amplifiers PA70
The 4 element yagi is very lightweight and easy to lift onto a telescoping tower. Due to the relatively thin elements its wind load is low and in my setup it has an SWR of less than 1.2 on 70.154 MHz without tuning. I lack the equipment to measure radiation patterns, but when targeting stations, the antenna’s directivity meets my expectations for a 4-element beam.
In all, the PA70-4-1.5B makes a good first impression and is lightweight enough for balcony operation. The antenna’s boom is 1.47 meters and the reflector element 2.17 meters long.
Load coil tuning a 5.6 meter dipole to the 15 meter ham radio band.
Setup for a ham radio vertical dipole of 5.6 meter fishing rod length that works multi-band 40 to 10 meters with inductive loading. No need for radials because of K9YC style ferrite chokes. Design suits restricted space balcony or portable operation.
As a regular K9YC vertical, the system covers the 17 to 10 meter bands using a transceiver-side antenna tuner. However, the proposed asymmetric loading technique achieves far superior performance on the 17 and 15 meter bands. It moreover enables operation on the 20, 30 and 40 meter bands within the 5.6 meter size limit. Just by placing load coils off-center the design results in good SWR matches below 1.5.
Due to its lightweight construction, all it takes to support the vertical antenna is a telescoping fishing rod. An additional telescoping tower made of aluminum or steel lifts up the heavier ferrite chokes and lower end of the dipole.
SWR Matching of Vertical Dipoles Shortened by Inductive Load
It is well known to the ham radio community that shortening wire antennas by inductive loading reduces the feed impedance. For instance, the ARRL Antenna Book[1] has impedance tables for top and base loading of verticals, according to which the center feed impedance of a dipole shortened by factor four drops to below 5 Ω. To counteract this effect, moving away from the center increases the resistance of the feed point. Therefore, a combination of shortening by inductive loading and shifting the feed point adjusts the feed resistance to a wide range of target values, such as the 50 Ω commonly in use by ham radio transceivers.
For a better understanding of asymmetric designs I recommend checking out off-center-fed dipoles[2] like the Windom[3] antenna.
Because of the design of vertical dipoles with high impedance ferrite chokes, matching antennas by off-center feeds is especially unproblematic. Unlike the Windom[3] antenna, asymmetric vertical dipoles do not produce a heavy load of shield currents.
Shortened Vertical Dipole Wiring Scheme
Figure 1: wiring scheme for shortened dipole with RF choke and antenna sections.
Figure 1 shows the wiring scheme for the shortened dipole with RF choke, feeds and inductive load. Labels L1 through L4 identify the antenna sections for which dimensions are given in the next section. As explained in my post on K9YC Verticals, the RF choke serves to electrically separate the dipole from its feed.
Vertical Dipole Antenna Wires with Loads and Dimensions
Figure 2: coil sections of vertical dipole antennas for 40, 30, 20 and 15 meter amateur bands.
Figure 2 depicts load coils for 40, 30, 20 and 15 meter ham radio bands. All coils are fixed on the same kind of tubing for electrical installations available at hardware stores. The tubes are 2.54 cm (1 inch) in diameter and the enameled copper wires have thicknesses of 1.5 and 1.2 mm. Coils for 40, 30, 20, and 15 meters have 97, 55, 25, and 9 turns, respectively. In addition to the coils, the image also shows the RG 58 feed lines with just the inner conductor connecting to the L2 wire sections.
Since all the L1 feed sections are about the same length, it’s surprising that the L2 sections don’t vary much either. This is very likely not optimal and when I find the time, I may experiment some more.
Nevertheless, despite the similar wire lengths, in all of the configurations the feed point moves away from the center electrically. This is due to the growing portion of the full-sized dipoles that the inductive loads replace. As a result, the center point of the dipoles moves inside the inductive loads. This effect, in turn, electrically pushes the feed point towards the lower side of the antenna. So as the center impedance decreases with larger coils, the cancellation effect of the off-center feed point increases. At least, that’s my theory.
Having verified that my full-size vertical dipoles worked well, I actually expected to need impedance transformers on top of the load coils. But as is, the system performs much better on lower bands than I had hoped for. All the same, I think it might be worthwhile to optimize my dipoles with some modeling software.
Wire Lengths
Band
L1
L2
L3
L4
10/11m
2.37
n/a
n/a
2.41
12m
2.725
n/a
n/a
2.74
15m
2.60
0.21
0.035
2.505
20m
2.61
0.19
0.07
2.47
30m
2.63
0.17
0.125
2.42
40m
2.62
0.19
0.16
2.365
Table 1: Wire and coil lengths with labels as in Figure 1. All in meters.
Table 1 has the wire lengths for my configurations for 10 to 40 meters. The L1 numbers include the 12 cm from SO 259 plug of the wires to the first ferrite choke. Between 26.985 and 28.074 MHz I get an SWR of 1.6 or below. For 12 meters I used a low loss feed cable so I can match the wire between 18 and 30 MHz with an antenna tuner. However, on 21 MHz tuning losses are already quite significant. Setups with load coils for 15 to 40 meters are likely not optimized, but way better than the alternative of using a match-box. As mentioned before, it might still pay off to verify dimensions in a modeling software.
Figure 3: the setup for 15 meters rolled up.
Mounting and Changing Dipole Wires
One obvious downside of the verticals is the need to swap wires in order to change bands. But with some practice, the process is now relatively swift for me.
A simple knack to get wire on and then the coil over the rod is to first pass the end of the wire through the coil like in Figure 4. Then slide the coil over the black tip of the rod on the left hand side to get the end of the wire on the right hand side of the coil. Afterwards, with the rod pointing up vertically, push up the rod starting with the thinnest innermost segment. In the process it’s advisable to rotate segments to get the wire wrapped around the rod.
Figure 4: Pass the top end of the dipole back through the coil to fix it at the end of the fishing rod.
Also note the paper clip that I keep attached to the top of the rod with the white cord. It’s very convenient because the little ring that secures the wire quickly slides over. Keeping the clip tied at all times has the huge added benefit of not having to go fishing for the tip of the rod. Hold up the rod vertically and try to get out its smallest segment to see what I mean.
Figure 5: Tip of the outer segment of the rod with paper clip attached and rubber cap holding the cord in place.
When flipping wires I always keep the fishing rod’s rubber cap at hand. Not only does it protect the outer segment from damage, it’s also perfect to firmly hold the paper clip when attaching wires.
Tower and Rod in Operation
Figure 6: collapsed telescoping tower with multi-band RF choke and extended fishing rod.
Finally, my telescoping tower on the left and extended rod on the right side of Figure 6. I step on a ladder to clip in wires with RF choke and rod mounted on the collapsed tower. Note how the coil for the 40 meter band, pointed to by the orange arrow, hardly adds to the wind load of the rod. Similar for the ferrite chokes with a much smaller wind surface than an equivalent T2LT coil for 40 meters.
Amidon FT-240 43 ferrite choke with 50 Ω microwave cable.
K9YC style verticals separate center-fed vertical dipoles from their coax feed lines by means of high impedance RF chokes. First, a short summary of Jim Brown’s original idea[3] and its mention in the ARRL Antenna Book[5]. I then describe my experiments in developing the high impedance broadband RF chokes that make this type of antenna possible.
The K9YC Vertical Dipole
The two halves of K9YC vertical dipoles consist of the shield of a coaxial feed line, and a wire connected to its inner conductor. At first glance they look like ground plane verticals without radials. But that’s because K9YC verticals are dipoles that don’t need radials as counterbalances.
Figure 1: a K9YC vertical dipole made up by shield and extended inner conductor of a coaxial feed line.
Figure 1 has an idealized K9YC vertical dipole with feeding point in the center, as indicated by the two λ/4 halves and amplitudes of RF currents dashed in red. This configuration is equivalent to a perfect center-fed half-wavelength dipole. But unfortunately, it cannot be connected directly to coaxial feed lines without altering antenna characteristics. Effectively, a coax feed bringing a signal to the lower end of the dipole will turn the configuration into a quarter-wavelength vertical lacking radials.
K9YC’s solution to keeping the structure electrically isolated as well as connected to the transmitter line is to place an RF choke at its lower end. Figure 2 shows a schematic of the choke, which allows signals to pass from the feed line while blocking RF currents from the shield.
Figure 2: wiring diagram having an RF choke at the bottom of the lower end of the vertical dipole.
Note that the K9YC design is quite similar to the T2LT. Instead of ferrite chokes the T2LT uses coil of coax feed line as a choke. To me the 8 to 12 cm diameter coils look like their impedance might well be boosted by resonance. If someone tried this, please fill in the gap as a comment.
Before diving into details of constructing high-impedance RF chokes, some words on why radio hams might find the proposed vertical dipole useful. After all, the construction looks weird and is likely to be met with a good deal of skepticism. Some say it couldn’t possibly work at all due to a lack of radials. Others give it the benefit of doubt, but state that a regular ground-plane would still perform better. I feel such criticism is missing the whole point of using K9YC verticals.
First, vertical dipoles, including many popular CB antennas, are great solution for installations without room for radials. Second, ground-plane antennas only perform well with several radials at right angles. However, most ham ground planes work with only one radial per band. Third, K9YC verticals shift the feeding point half an antenna length up, thus gaining all-important height.
Broadband high impedance RF chokes can be implemented by winding coaxial cable through a ferrite toroid. Such chokes make use of the shielding effect of coaxial cables to separate signals from unwanted shield currents. For signals, the magnetic fields of the shield and the inner conductor cancel each other outwardly. Therefore, the ferrite choke will not attenuate signals between transceiver and antenna. In contrast, shield currents are not canceled out and therefore induce magnetic flux in the ferrite. This fact is well known to the ham community building cable trap baluns.
In summary, RF currents from the lower end of the dipole will “see” the coil as a resistance. However, the signals between transceiver and antenna are only subject to normal cable attenuation along the length of the cable wound through the ferrite.
Appropriate Choice of K9YC Broadband RF Choke Ferrite Material
Creating broadband high impedance RF chokes, I experimented with Amidon FT-240 43[1] and FT-240 77[2] ferrite toroids.
Figure 4: Amidon FT-240 43 with regular copper wire for impedance measurement.
For the FT-240 43 the AL value from datasheet[1] is 1075 nH/turn, note my factor 106 conversion milli to nano. With 10 turns like in Figure 4 the trap coil should have an inductance of 107.5 μH.
L = 10^2 * 1075 * 10^-9 H
L = 1075 * 10^-7 H = 107.5 μH
Thus, at 7 MHz, the ferrite trap from Figure 4 has a theoretical reactance of XL = 4.7 kΩ. At 28 MHz, the reactance would be as high as 18.9 kΩ. See Appendix 1 giving some theoretical background.
XL = 2 * π * 7 * 10^6 * 107.5 * 10^-6
XL = 4728 Ω
In my installations, I aim for a reactance of at least 10 kΩ over the frequency range of 7 to 28 MHz. Then, theoretically, two FT-240 43 trap coils of ten turns each should suffice for K9YC verticals. Believing the datasheets, traps with FT-240 77 should work even better. Given their AL of 2590 nH/turn, at 7 MHz a 10 turn coil has a reactance of 11.4 kΩ. So one might think that a single coil should safely cover the entire frequency range.
However, measuring the actual resistances of coils, as shown in Figure 4, gives different results. I found that FT-240 77 is good for frequencies below 7 MHz at best. Vertical dipoles with RF chokes using FT-240 43 have much sharper resonances and perform noticeably better.
Practical Choke Implementations for Telescope Towers
Figure 5: FT-240 43 chokes in serial connection mounted to fishing rod, plastic tube and aluminum telescope tower.
Figure 5 shows my approach to mounting the K9YC RF choke on an aluminum telescope mast. The fishing rod serves to pull up the vertical dipole halves. A plastic pipe for electrical installations holds three series-connected inductor coils in a straight row. Installation clamps connect the rod, plastic tube and aluminum pipe.
For the Amidon FT-240 43 material I found no reasonable upper limit of turns using thin Sucoform[4] cable. I could fit 17 turns and estimated no decrease of impedance due to capacities or resonances on 28 MHz. I should mention that I am lacking dedicated equipment for measuring RF impedance. Presently, I am using a dedicated measurement circuit and a digital oscilloscope for this purpose.
Using expensive Sucoform microwave cable is probably overkill. Reduced capacitance and small bending radius aside the stiffness of this cable produces chokes of better mechanical stability.
Figure 6: packaging, rain proofing and mounting detail for the FT-240 43 RF chokes.
To protect them from the rain, I wrapped the ferrite cores in a freezer bag with the bottom open.
Figure 7: mounting detail for tower to tube and fishing rod connection with SO 259 socket.
I use installation clamps, available at most hardware stores, to attach the tower to the pole and also to attach the SO 259 sockets.
Figure 8: FT-240 77 variant of RF choke using regular RG 58 cable.
Figure 8 shows my other experiment building the K9YC from a serial connection of four FT 240-77 coils. This trap will be good for 160 m and medium wave antennas. While the Amidon 77 material is not well suited for frequencies above 3.5 MHz, the RG58 cable seemed to do the trick. I achieved highest impedance fitting 15 turns of RG58.
Figure 9: electrical installation tubes slightly larger in diameter ensures equal distancing of choke coils.
Finally, Figure 9 shows how plastic tubes from hardware stores with slightly larger diameters equally distance the four choke coils. This implementation is a bit over-designed for the kW power range. My other chokes are tested for up to 100 W output power.T
Short Anecdote how I came to use K9YC Verticals
I stumbled upon Jim Brown’s design reading the ARRL book after a stay at my parents’ house, following a sked with an old friend from CB times. As a 12 year old kid, my electronics and radio hobby started with partly home-made CB radios not too compliant with the strict German regulations of the time. Our equipment produced maybe 4 W of RF power, which was well above the 500 mW that Bundespost set as a legal limit for the legendary 12 channels AM. Looking back, it seems surprising such crappy transceivers allowed for stable connections in our small-town. Especially since my friend and I had trouble hearing each other on 21 MHz with modern ham-radio transceivers, ground-plane antennas and 100 W PEP in SSB.
So the much ridiculed CB technology must have done something right! And my only explanation is that our CB vertical antennas must have performed far superior to our amateur radio ground planes. In fact, the reason for the poor ham performance is painfully obvious. Multi-band ham radio verticals typically skimp on a single radial per band, turning the verticals into V-shaped dipoles with irregular radiation patterns.
For CB, we used either end-fed half-wave vertical dipoles, or 5/8 λ verticals. And while my “City Star” half-wave dipole performed slightly worse than 5/8 λ, it had the huge advantage of not requiring space for radials. Exactly the problem I still have as a big city apartment dweller!
Appendix 1: Computing Trap Coil Reactances
Calculating the expected inductance of ferrite toroid trap coils is straightforward. Simply multiply the AL value from datasheets with the square of cable turns!
L = AL * n2
L: coil inductance (in Henry)
AL: ferrite specific inductance per turn
n: number of turns
The impedance, or roughly resistance to shield currents, is now given by the well known formula for the reactance:
XL = ωL = 2 * π * f * L
XL: reactance (~ resistance)
π: ~ 3.141
f: frequency in Hz
L: inductance in H
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