CAT 5e Switched Dipole

Phil, AD5X, came up with a great all HF band dipole.

http://www.ad5x.com/images/Articles/Dipole%20RevA.pdf

He used single pole double throw switches to create a resonant dipole for each band 40-10 meters. The whole antenna is a full-sized 18 gauge dipole from 10-20 and the 30/40 meter inclusion simply inserted inductance in series with each leg across an open switch fed with RG 174 coax. A great antenna but I wanted to see if I could reduce some weight.


While I haven't created the necessary inductance to get to 40 meters, the result of my fiddling has yielded a full-sized dipole that is resonant on 6, 10, 12, 15, 17, 20, 30 meters and fits inside a quart-sized Baggie using a continuous length of twisted pair from a piece of CAT 5e cable.


Now, when I say "resonant", what I mean is very near 50 Ohms at the antenna terminals, which is an important distinction when using CAT 5e twisted pair. Its characteristic impedance is 100-Ohms and something inside your brain says that it won't be matched to a 50-Ohm system. You are correct but I will show you the way to make this as efficient as possible using an antenna tuner at the rig, and a switchable mini-balun, with the payoff of having a 3 ounce, 6-30 meter antenna system for your shiny new QRP rig, while only sacrificing 0.8 dB at the most!


The worst case is, in my case, on 10 meters where the EZNEC calculated Z is 42 - j37 Ohms. The VK1OD twisted pair calculator reveals that when a 42 - j37 Ohm load is placed at the end of 20.5 feet (6.25m) of twisted pair line you get 0.8 dB of mismatch loss in addition to 0.5 dB matched line loss for a total loss of 1.3 dB at the worst case frequency and worst case load impedance.


An 82% efficiency at the worst possible load and frequency, 10 meters, in my case. If you payed closer attention to getting 50 Ohms on all bands the only loss incurred would be the loss of the Tx line, balun and tuner.


Now, notice that the input impedance (Zin) is very near 200 Ohms which is a very efficient match when you switch the mini-balun to the 4:1 setting.


Adding up, systems losses in this case yields 0.5 dB line loss, 0.8 dB mismatch loss, 0.2 dB balun loss, 1.5 dB total system loss! A very acceptable trade considering portability, cost and efficiency.

A closer examination of the Smith chart reveals a neat relationship between a 50 Ohm source, 100 Ohm Tx line and 200 Ohm load. A reactive transformer (Q-match) is formed and used to our advantage. No matter where your impedance comes up with varying lengths of Tx line, the input Z will always be between 50 and 200 Ohms, an easy match for the switchable mini-balun attached to any auto tuner.


Now all that's left is to build one. If you pull the rip cord found inside most cables, you'll be able to free the pairs fairly quickly, the hard part is un-twisting the pair to your highest frequency and cut the remaining inter-switch wires to length and unravel those separately. BTW, don't use Phil's lengths in his document because they are inaccurate, use the 468/f(MHz) formula and you'll be fine.

73,
Myron WVØH

Not a Loop

Since the KX3 arrived, I have been playing with the idea of a "Grab n Go" rig for quick deployable, field expedient HF system.

The obvious is a vertical wire and single counterpoise which is the quickest but comparing EZNEC plots of ANY vertical to a dipole, the dipole will always win over our poor Colorado ground. The dipole is quite a bit better than the magnetic loop antenna described earlier.




I have been searching for a quick deployable dipole capable of operating 6-40 meters. My initial response was putting two 46 foot, #26 PVC hook up wires together in a dipole configuration that made for a 50 foot dipole and about 23 feet of twisted pair feedline. Placing a tuner at the business end one could expect to tune the beast to any frequency. And it did. But. I had forgotten about the loss induced by a high SWR on the twisted pair transmission line formed by the PVC hookup wires on a frequency not naturally resonant to the dipole length chosen, such as the fundamental frequency of 10MHz or the third harmonic of 30 MHz. If I had chosen such frequencies the loss due to SWR would not be a factor just the loss of the PVC transmission line. Well, what is the loss of PVC wire? Well, 100% PVC has a loss tangent of 0.016 at 1MHz and 0.0081 at 100MHz, compare to 0.0002 of Teflon.


So after some thought about the significance of the loss tangent and comparison to Teflon, I concluded that the PVC wire is just too lossy to satisfy my quest of putting up an efficient dipole. VK1OD has a great website dealing with twisted pair transmission line loss and other loss calculators as well.



For any given length of twisted pair transmission line, the major loss factor will be the material that is mostly between the conductors and the resistance (AC/DC) of the conductors themselves. For example, a twisted pair will have loss associated with the dielectric and to a lesser extent the resistance of the wires, if they are reasonable size i.e. bigger than 26 gauge wire.

Well, this meant that my idea of using chalk line reels for storing each leg of the dipole was becoming less viable the more I looked into this loss mechanism. The key was to find a dielectric that would be good to use, Teflon, perfect, but I didn't have any appreciable lengths of Teflon wire to make a multiband doublet.

Enter junk box. What have we here? Some Plenum Cat 5 cable. Could this have some twisted pair that could be used?


If you take a look at the attenuation column you will see that the loss is rather high for a given frequency. Or is it? The fine print tells us that the loss is for 100 meters (328 feet)! Well that makes me feel better as we are only talking about runs that are 30 feet or so in length which means this number divided by 328 and multiplied by 30. Well, so where does that leave us? How does it compare to say RG-174 or RG-58? After some network analyzer measurements at work with two new baluns calibrated to 100 Ohms, I have found the loss to be comparable to RG-58! And it's balanced to boot. Well. You say, "don't get it close to other metal or you'll loose all of your signal". Really? So I tested that theory as well.


While not a completely false statement, it was not as bad as feared. If you roll up 40 or so feet into a spool and check it's insertion loss, you'll notice only 1 extra dB or so has dissipated into the surroundings. Only 1-dB!

If you parallel the transmission line to some metal, I cannot make it loose any RF's, at least not measurable by the network analyzer anyways.

Bottom line, lay it on the ground, run it next to coax, you can even leave it inside the sheath that it came in. Oh yeah, why do't they worry about loss when it's placed side by side in a 4 pair bundle? That's because they don't worry about that as much as wee hams.


Next blog, I will talk about the intricacies of using the Cat 5 cable as a dipole.

73, Myron WVØH

Trail Friendly 4:1 & 1:1 100 Watt Balun

When the Elecraft BL2 balun arrived I was struck by the simplicity of how using a DPDT switch could be wired in such a way to allow a Guanella current balun to be used in a 4:1 or 1:1 mode. Those Elecraft guys are ingenious. The BL2 being rated for 250 Watts, I wanted one that could handle 100 Watts at most and 20 Watts or so easily and way smaller. I borrowed an identical balun from WB9QDL, Ken and measured the back to back insertion loss as a baseline.


So I decided that a smaller version was feasible after seeing how "big" the BL2 really was.

I started out using smaller 2402 sized binocular core with type 43 material because I had a box full from a BARCfest.


Using some old Polyethylene wire I had laying around, precut I might add, I twisted the wire together to get close to 100 Ohms, the geometric mean impedance for correct transformation of 50 to 200 Ohms. This sparked another debate over the loss characteristics of various twisted pair transmission lines. This will be another post. Anytime you twist some insulated wire together you'll end up with an impedance between 95 and 150 Ohms, according to an Agilent document I found on the internet. It looks like old telephone cable with a black outer sheath and corrugated copper sleeve.


Using the cordless drill I twisted two wires together.


Following the Elecraft winding technique of winding from lower to upper holes three times in the lower holes and two turns in the upper holes.


The finished configuration allowed the 100 Ohm twisted pair to connect in such a way to provide 50 Ohms when in switched to parallel, 1:1 mode and series connected output's in the 4:1 mode.


I made two balun's, tested them back to back, divided the insertion loss by two and obtained decent results.


My hesitation to build the balun's with these smaller cores was the fact that there wasn't sufficient choking reactance on 40 or 80 meters to prevent excess common mode current from flowing though the balun. Since I really wanted the performance to be mostly equivalent to the Elecraft model, I needed to extend the frequency response downward without sacrificing the 6m performance. So I ended up selecting the larger binocular core size of Amidon 0202-73. This material was great at providing the sufficient reactance at low frequencies but its high permeability caused excessive heating when applying upper HF power. So I turned to the same sized core in a type 43 material and that's where I ended up. Here is the parts list:

12 inches of a twisted pair from PLENUM rated Cat 5e cable ~100Ω (FEP). Don't use the brown/white non-FEP pair, I don't know it's loss tangent. FEP loss is really close to PTFE loss.
2 Amidon BN-0202-43 binocular core ferrites
Radio Shack banana jacks, 274-0661
2 each 1/4" O-rings (junk box)
Radio Shack slide switch, 275-0403, I like this one because of its larger size, less capacitance
Panel mount BNC jack (next ones will be male BNC to eliminate short coax jumper or adapter)

Cut the twisted pair into 2-six inch lengths and thread them through the bottom stacked cores, up around to the top and through the bottom again for a total of three times through on the bottom and twice through the top. A drop of crazy glue will hold the cores together whilest winding.

Drill a larger than required hole for the banana jacks to keep excess metal from creating too much capacitance to ground. I used a 1/4" drill bit and put in a 1/4" O-ring to keep it centered. (I did not use any metal washers or the metal solder tab but rather Dremeled the threads of the post and soldered directly to them.) Crazy glue the black fiber washers to the terminal binding post then set the O-ring in the hole and glue the post in the hole with the washer, then place the second black fiber washer over a puddle of glue around the other side and screw it down. You may want to use slow setting gel type glue as the instant type made me plan in advance and work quickly. Basically, gluing all sides of the post/washer to create enough friction to prevent the metal post from twisting, because it will twist, it's really a poor design.


Since building two baluns this meant having two separate mode switches, I found a 4P4T switch in my junk box which ideally allowed the process of switching modes from 4:1 to 1:1 simultaneously.



After having a conversation with KN5L, John, I set out to test the common mode performance by grounding each side with a clip lead to see the rise in SWR. It faired better than I initially thought. 1.5:1 SWR rise occurred well out of the 80 meter band on the low end and above 70MHz on the high end, so I was happy. Note: if used on 160m, this balun would still work, it's just that any differential mode currents would be a bit higher than normal and not really be a big deal.


The balun did very well with the max insertion loss being in the neighborhood of 0.3 dB at the frequency extremes, and slightly lower in the center. My testing at 100-Watts was done using my Kenwood TS-2000 with a Bird 43 and 100H slug. So the only thing left was to incorporate this whole thing into a trail friendly package robust enough to handle the rigors of trail use.


What followed was a bent piece of Aluminum, one fairly large DPDT Radio Switch superglued down to the Aluminum and some low profile Radio Shack binding posts. I cut off the mounting hole tabs of the switch and shaved down the plastic switch actuator to make it more low profile. I used a female BNC panel mount connector but realized that a male BNC would eliminate an adapter. All in all, the whole thing weighs around 45 grams and doesn't seem to take up much room in the pack.

8 foot mag loop, 2 days on

I put the loop on a 2x2 with a 1x2 cross frame.





It was tested on the South Dakota Emergency Net on 3960-kHz Sunday morning. My reports between the loop and the wire were promising. As predicted, the signal was about 6 dB weaker than the wire. 1-S unit!





I made a contact on 40 and this antenna really works well there. I worked a guy in Durango at 3:30-4:00pm today and it really has good NVIS properties.






Hope to hear you on.





72,
Myron

Results of a CQ Run

Here is a quick test of the 8 foot loop. It will tune from 3.4-14.4 MHz (whew, squeaked by on that one.)

It will put out a signal on 20, 40 and 80 meters (haven't tried 30m).
Here is the results of a CQ run.




I think that the overall RBN performance is better than the 4 foot loop by 5-6 dB, which holds pretty close to predicted. So this loop should only be an S unit down from my HF wire.

Myron

The Finished 8' foot mag loop

Well. I made it! This is the finished magnetic loop that is a scoach under 8 feet in diameter.




There is a tendency for the antenna to have better SWR when the smaller loop is closer to the outer coax for the higher frequencies, as I have found with the 4 footer. When I go down to 3530 kHz the coupling loop tends to end up away from the outer loop as shown here.




This shot of the RBN shows a promising result on 40 meters after just a few CQ's. I did try to call on 80 CW but no spots were returned in just a few attempts at a CQ. More results later.

So the overall loop is just under 8 feet and the input coupling loop is 60 inches in circumference (8/5*12).





Now on to the CQ-ing!

Myron

8' diameter mag loop

I set out to modify my 4 foot loop and come up with an 8 foot loop that will make 80 meters more of a reality.

Already successfully using the LDF4-50 (1/2" hardline) for the 4 footer, it was only appropriate to try using it for the larger design. If I could increase the size of the loop the radiation resistance will go up. After running the numbers on the AA5TB spreadsheet, I can gain about 10 dB from the doubling of the size.




I first tried the multi-turn approach but the Q was so high that the tuning sensitivity was unmanageable, it ended up being too squirrelly.


The interwinding spacing was very sensitive to any movement! That was that. I need to make it a single turn with an 8 foot diameter.

Next blog post, the finished product.

Myron