By Morgan Bailey NJ8M
Having wires strung all over the place on a city lot is not something that the neighbors like seeing done. Keeping the landscape as clean as possible is a good thing, both from a real estate perspective and from an RF interaction consideration between antennas. Also, small lots many times present challenging problems to solve in size and visual impact. Using less wires and giving the amateur operator more bands can be solved by making wire antennas a multiband approach. My solution that allows me to run 1500 watts without problems is to construct an inductively loaded dipole. Alpha Delta antenna products have many products that do exactly that, but why do they work and can they be designed for other frequencies? The following is, I hope, a simple presentation of how to design any dual band antenna using coils to make it fit on your lot. I want to site an excellent website by K7MEM.COM. He has done a bunch of work on shortening antennas but I wanted to take it a simple step further and present my reasoning and how to design any dipole to be a dual band antenna, especially on 80 and 40. This seems to be the problem for most amateurs to solve.
Questions arise as to how to proceed without using engineering approximations which falls under the heading of a Wild-Ass Guess. Cut-and-try engineering will work but it takes time and many don’t have the patience to succeed. Hopefully, my approach will cut these approaches to a minimum. Problems with inductively loaded dipoles are mainly the ease of construction, the location of antenna and narrowing of bandwidth of the second band. The use of an antenna tuner, matchbox, to increase bandwidth, predicates not using a center balun. High SWR will toast a balun. For this reason AlphaDelta does not recommend a balun and specifically states in their literature that is shipped with the antenna that the “the antenna is designed to not be used with a Balun.” Many would argue that without a balun, the pattern will be changed, specifically, on 80 and 40 meter bands. This is a non-issue because very few amateurs have the ability to put a dipole at 60 feet for 40 meters, much less 120 feet for 80 meters. So, mainly the city dweller will have an NVIS antenna with a center fed dipole up 30 feet and because, most amateurs don’t have 3 supports to make it a flat top, will be using the inverted Vee approach. This is just fine because it gives an omnidirectional pattern and lowers the feed point from 70-72 ohms down to 50 ohms which is what RG213 and RG8X, RG58 and LMR400 coaxes are designed for. Putting up a dipole with an included angle less than 90 degrees falls under the, “ Any antenna is better than No antenna,” category. In general, put a dipole up as high as you can and make the included angle greater than 90 degrees and you will be a happy camper. Don’t worry about DX, I have worked DX from Kansas using an inductively loaded dipole up 20 feet. Is it the best choice, no, but if it is your only choice, you are good to go. Now for the theory, as a general class op, you will have already seen the theory in the test question pool. We will use 2 formulas from that and a third for calculation of an inductor based on turns per inch, wire size and length of coil.
The first formula is the frequency of a half wave dipole:
468/frequency in Mhz = Length in feet of a half wave dipole
This is the pure theory that proves out. Variables are added when you don’t use bare wire. The use of THHN wire from the big box stores is the common go to for antenna building. I use #12 black insulated THHN wire that comes on the 500 foot spools for around $50-60. The insulation adds a velocity factor that shortens the antenna. This is not a factor because trimming the antenna to resonance will be done empirically based on the location and installation of the antenna. In general, it is better to cut longer because it is easier to trim off rather than add because it was too short. I generally add 4 feet to the measured length for 80 meters and 3 feet for 40 meters. This gives me plenty of length to resonate by trimming the length and to terminate the ends and center connections that are necessary. Because of the velocity factor of the wire, it will be longer. Add the length any way. In this case, more is better! That takes care of the basic dipole construction of a single band antenna. Now let's add a second band by using a coil, inductor, and shortening the lower frequency length but keeping the higher frequency un-affected.
Any dipole can be made into a 2 band dipole by trapping the higher band with an inductor and thereby shortening the lower band length. The added inductance at the higher frequency shorts out the longer wire after the coil. At the higher frequency of the dipole the center dipole does not see the wire beyond the coil. At the lower frequency the inductor adds inductance to the dipole, RF flows through the inductor and by trimming the tail of the antenna, it can be made to resonate on the lower frequency with a greatly shortened length being made possible, thereby making it a dual band dipole with a single feed point. This cuts down on coax runs and decreases cost to the new amateur. So how much inductance does it take to make this magic happen? We need to address how much resistance at a given frequency of operation is necessary to present a resistance to flow of the RF in the wire. I want to simplify the theory of operation by using common sense, which in today’s environment, is rather uncommon. Let us look at data that is easily obtained without any experimentation but just by reading the specs of a product already in production. That being a choke balun used on coax to stop the flow of RF on the shield of coax. Going to BalunDesigns.com and looking at their graphs for these baluns it is found that 3500 to 4000 ohms is enough to get the job done. That was easy. Now lets build a coil that has inductive reactance at that frequency giving the 3500-4000 ohms necessary to get the job done. Ok another formula:
Inductive reactance in Ohms = 2 x Pi x Frequency x Inductance
Yes we can solve the equation, but it is easier to just go to a web site, plug in values till you get the right resistance needed, 3500-4000 ohms, and get your answer easily. One such site is:
It will give you 2 boxes to fill in. For an 80/40 dipole, you will want to stop the RF of 7Mhz from going beyond the inductor/coil. Entering the frequency of 7Mhz and a random inductance to play with to get to the desired 3500 ohms of resistance/reactance necessary to make the coil, one arrives at 80 microhenries. Because this equation is linear, that is, it scales directly, then by this logic a 20 meter coil will take 40 microhenries and an 80 meter coil will take 160 microhenries to provide the necessary values to obtain the 3500 ohms needed to isolate the end wire from the center dipole. Using a smaller value of resistance, 3000 ohms, and even using 3500 ohms will sometimes cause an interaction between the shortening of the end wire changing the resonant frequency of the center dipole at higher the higher frequency, and vice versa. If one goes to 4000 ohms this does not happen. It is like a switch that is an open circuit to the distant piece of wire. This makes resonating the dipoles easy as they do not talk to each other. First trim the wire on the center dipole then trim the end wire. This will greatly shorten the dipole length on the 80 meter band to around 80 feet over all. The band width will be lower, maybe about 40 Khz on 80 with 200 Khz on 40 meters under 2:1 SWR.
How does this shortening affect the radiation of the dipole? Since the main current of a dipole is radiated from the center, that is, the current is highest at the center of the dipole and lowest, going to zero at the ends, it has little noticeable effect to the receiving station. Simply stated the receiving station will not be able to tell the difference between 100 watts and 70 watts radiated power. It works just fine. This effect only applies to the lower frequency dipole. The center or higher frequency dipole is unaffected because it, itself, is not shortened. Only the lower frequency is shortened. To decrease this effect of shortening the length of the lower frequency portion of the dipole and increasing efficiency, one can use a value of 3000 ohms and still produce a 2 band dipole, but there will be interaction when you trim either the center or the end wires on the respective dipoles. I settled for the middle ground and my 80/40 dipole is 108 feet long, which is down from 133 feet. It is between 80-90 percent efficient. It fits in my lot. I’m good with that.
Now for the last equation. How do we make a coil with the given inductance needed to isolate the higher frequency from the lower frequency one? The formula is involved:
Single Layer Coil D L One Turn - N D2 × N2 L (uH) = ------------ 18×D + 40×L Where: D = Mean diameter of the coil (inches) N = Number of Turns L = Length of the coil (inches)
Life is too short to do this math. Just go to the web page:
Plug in the values of inductance, the wire size you have on hand and the coil forms available for the PVC that you have on hand and you are good to go. Happy coil making.
I have made many coil loaded dipoles and here are some Pix of the coil construction:
These coils are made for 3.5 Mhz. They were used on a 160/80 meter dipole. Later one of these was used on my home station in the construction of an 80/160 meter Inverted L which is currently in use in my backyard. They are made with regular 3 inch PVC which measures 3.5 inches OD and wound with 14 gauge magnet wire. They laugh at 1500 watts RTTY.
Once the coils are wound a liberal coating of liquid electrical tape is applied. While this is wet, Scotch 88 electrical tape is applied with over lapping 1⁄2 to 3⁄4 width. Doing this while the liquid electrical tape is wet sort of vulcanizes the tape and makes a very solid construct that is highly resistant to abrasion. It is weatherproof and UV resistant.
The finished product is on the right with the treatment of liquid electrical tape and Scotch 88
tape. It is ready to be installed. The Inductance did not vary before or after the protective
treatment. It still measured 174 microhenries.
I usually cut and trim the lower frequency dipole for the phone portion of the band I want to
operate. This makes it way too short for the CW portion of the band which is my primary
operating frequency. To solve this, I add an end insulator made out of cheap Sams Club cutting
See below image. I use heavy 10-24 hardware on the insulator. It is what I had and the wing
nuts with a lock washer are easily removed. Using a phillips screwdriver for the phillips machine
head stainless steel bolt helps facilitate tightening the pigtail to the insulator.
This way I can add a pigtail that dangles to adjust the frequency downward. I also use this same
construct if I know that I am going to be moving the antenna and will need to re-resonate it at a
different location. Just soldering on a ring connector and bolting it to the nut makes easy work of
frequency change and tuning.
Both my son, NS0R and I have used these antennas for years and there have been many 10s
or thousands of QSOs on them running between 100 watts to 1500 watts with no problems. I
made one for Field Day which my son cobbled on to and uses at his home QTH on 80 and 40.
We later added a second wire from the center conductor for 20 meters and made a fan dipole
out of it. No interaction was noted by this modification. This is his only antenna. Using this
construction technique at my home station in the creation of a 160/80 inverted L, has enabled
me to get on 160 meters from my city lot. If you have questions:
Morgan Bailey NJ8M
Cell: 785-554-5561 24/7
( If I don't answer right away, I am probably listening to the bottom end
of 40 with my headphones on.) es 73 Cheers!