Dave Arrich AD6AE

CARS Technical Writer

Balun Location Considerations for G5RV and Other Dipoles

There are four suggestions regarding where to place a 1:1 balun, line choke, or a 4:1 balun transformer.  Several sources suggest trying one at the coax/ladder line connection;  DX Engineering suggests putting it at the shack which would indicate a long ladder line run and a short coax to get into the shack and tuner; others don’t use any. A fourth idea would be to test and trim the ladder line. Below there are four good articles for those interested in doublet antennas.

I’ve read two sources regarding length of the doublet arms and the ladder line length that advise against using odd multiples of 1/8 wavelength at the lowest frequency of operation. Instead they suggest using odd multiples of 1/4 wavelength (Ref. I).

Common mode currents are formed at balanced (dipole or ladder-line) to unbalanced connections (coax feedline). Unbalanced feedline presents what is actually ‘three conductive paths to RF. The first path is the center wire; the second path is the inside of the shield – these two paths are desirable. The third path is not desirable. It is the outside of the shield where common mode currents are created that flow back to the ground at the transmitter, tuner or grounded entry bulkhead. Electricity is like water and always takes the path of least resistance.

There appears to be no “magic” number for the length of the ladder or twinlead section for the G5RV but like all doublets, there are restrictions (Ref. I).  If terminating into a coax feedline, the ladder line acts as a transmission line transformer of a specific length depending on the impedance and velocity factor of the matching section.

Here’s some very informative links on doublets from the ARRL and professional RF engineers.

Multi-band Doublet Antenna

Anytime you see an article written by R.L. Cebik, W4RNL (SK), read it.

Additional Information about the Multi-band Doublet

1. What is a Doublet Antenna? From Electronics Notes.
2. Introducing the “All-Band” Doublet (L. B. Cebik, W4RNL)
3. All -Band Doublet (L. B. Cebik, W4RNL)
4. Suppose I Could Only Have One Wire Antenna (L. B. Cebik, W4RNL)

Direct connections to coax

Although it is always best to use an antenna tuning unit with a doublet antenna, it is possible to operate the antenna on even multiples so that one may not always be needed. Ref. III

Any length of balanced feeder can be used with the doublet, the impedance match is best if the total length of one leg of the antenna and feeder, i.e. L1 + L2 equals an odd multiple of electrical quarter wavelengths of the frequency to be used. Using this approach, the impedance will be around 50Ω and mainly resistive. See the formula below to calculate length. (Ref. I)

If the low impedance option is used to directly match to 50 Ω remember that it will require a 1:1 balun in circuit to provide the required balanced to unbalanced transition and common mode suppression. However, it is always good practice to incorporate an antenna tuning unit to ensure that the transmitter is presented with the right impedance in frequencies that are not an even multiple of the design frequency, even if it is expected to provide a good match.

Why are they different lengths?  Mostly because of their minimal differences in characteristic impedances and velocity factors. 300-ohm twin lead has an average VF of 0.83.  450-ohm ladder line has an average VF of 0.92. 

Therefore, 29 feet of twinlead is needed for the equivalent delay presented by 32 feet for the ladder line.  Velocity factor is actually a measure of how much the traveling wave is delayed in getting from source to load.  The higher the VF, the longer the line must be for the required delay.

From qsl.net/w7nat/coax.html

Velocity Factor is used to determine the physical length of cable to be used for a specific wavelength, such as 1/2 wavelength. For antennas that are not resonant (SWR exists) it is advantageous to use a feed line that is an electrical multiple of 1/2 wavelength long.

The formula for determining the physical length of a half-wavelength of cable is:

L = VF x 492 / f

Where L is the physical length of one half wavelength, VF is the velocity factor, and f is the frequency of interest. Here’s the why it’s advantageous: Even multiples of ½ wave lengths, reproduce the load impedance. In the case of a non-resonant antenna, using this method is actually tuning both the antenna and feedline and is a really bad idea. A radiating feedline unless intended by design (as used in OCFD’s, actually forces unpredictable effects on a balanced antenna pattern and creates Rf in the shack. The feedline and transmitter should always be matched to each other and to the load.

Example of Velocity Factor:

The speed of light has a Velocity Factor of 1.0. This example will compare the delay for two different lengths of RG-8X. Velocity Factor is actually measured in nanoseconds per foot so I’m using the reciprocal of (1/VF) here to convert VF back to time in nanoseconds/foot.

For this example, two different lengths of RG-8X, which has an average velocity factor of 0.79, will be compared. Length “A” will be 100-feet. Length “B” will be 572-feet.

Velocity is Distance divided by Time (feet/second, miles/hour, etc.). So, dividing distance by the propagation delay factor where: for propagation delay for length A, would be 100/.79 or 127 nanoseconds; and for length B would be 572/.79 or 724 nanoseconds.

As shown above, the times for the signal to reach the load in cable A will arrive 597 nanoseconds before that of cable B. These delays can be used to an advantage. One application is used for 2, 3, or 4-element, phased-vertical, antenna arrays that can be electrically rotated to ‘aim’ the main lobe radiation pattern around to a desired coverage area. Switching different lengths of the same type of transmission lines to any two or all antennas will ‘delay’ the signal with respect to the reference antenna and steer the main lobe. It is similar to how a multi-element beam antenna works. They use varying fractions of an electrical wavelength for physical separation using the resulting phasing delays between the driven element and reflector and/or the driven element and director(s) to either reflect the incident wave; or reradiate it in the same direction to actually focus strengthen the traveling wave. Similar in operation to the multiple lenses inside a telescope.

A quick refresher:  The impedance of a standing wave of voltage and current on a resonant dipole antenna at a fixed frequency is lowest at the center and highest at the ends (E & I are 90° out of phase).  That is because the voltage is zero at the center and maximum at the ends. At the same time, current is maximum at the center and zero at the ends or a low impedance.

As you measure outward from center toward either end, the voltage and current amplitudes change. Voltage (E) is increasing while current (I) is decreasing. R=E/I.  

Note where this is going.  The further away from center you move, the higher the voltage amplitude and the lower the current amplitude, which results in a higher impedance. These extremes can vary from < 50-ohms to > 4,500-ohms or more.

The same holds true for ladder line.  It is a two-conductor linear transformer where the impedances change every ¼ wavelength as in the dipole above. As you move from source to load the standing waves exhibit the same characteristics.  Cut it just the right length (between multiples of a ¼ wavelength or 90°), and you can match a 50-ohm source impedance to a different impedance load for a given frequency.

Start with lengths that are ELECTRICAL odd multiples or a ¼ wavelength at the lowest frequency of operation.  A resonant antenna like a doublet as shown in the tables of the links provided, will be easier to tune and will operate well at even multiples of the lowest frequency for which it is designed without a tuner. The WARC bands will make the antenna appear as a non-resonant antenna and may likely require a tuner or transmatch. 

Even though the 300, 450 or 600-ohm matching section transforms those impedances down close to 50-ohms, there is still the problem where the feedlines transition from balanced to unbalanced and where common mode currents are generated. It’s an unavoidable law of physics, and can create problems.

This doublet installment was requested and not really a part of the series on TLT’s. However, common mode chokes will be discussed in detail in a future section of this series. “Everything should be made as simple as possible, but not simpler.” Albert Einstein.

To Mrs. Bixby, Boston, Mass.

Dear Madam,

I have been shown in the files of the War Department a statement of the Adjutant General of Massachusetts that you are the mother of five sons who have died gloriously on the field of battle.

I feel how weak and fruitless must be any word of mine which should attempt to beguile you from the grief of a loss so overwhelming. But I cannot refrain from tendering you the consolation that may be found in the thanks of the republic they died to save. I pray that our Heavenly Father may assuage the anguish of your bereavement, and leave you only the cherished memory of the loved and lost, and the solemn pride that must be yours to have laid so costly a sacrifice upon the altar of freedom.

Yours, very sincerely and respectfully,

Abraham Lincoln

By Dave Arrich AD6AE

In 1957 after acting on a whim, I discovered the answer to, “I wonder what the inside of a light socket feels like.”  It was love at first bite and has been my major focus since. I repaired my first radio at age ten and opened my radio & TV repair business – obviously on a very small scale – at age 13.  I was like a sponge and sought out independent repairmen who would tolerate a kid watching over their shoulder, asking lots of questions and relieving them of their inventory of old black and white TV sets.Antennas and transmission line theory has been a passion of mine from the beginning of my amateur involvement in 1972 after my Elemer and coworker at Fort Meade, Md. suggested that I try it.  From my first morse character to Extra only took a year.  Becoming proficient at copying 20 wpm with a stick is all that held me back.

In the spring of 1974, I got my novice license – WN3YSQ. Late that fall, my Elmer and I went down to the FCC office in Washington DC, where we were greeted by “Mr. Personality” who barked out some instructions and an hour later, we happily strutted out with our Extra tickets. My call was now WA3YSQ. In 1980, I completely lost interest and put my station in storage for the next 40 years.

When I returned to the hobby three years ago, my current vertical antenna required winding a non-standard ratio Unun that used an unconventional winding method. I had no idea on earth what I was doing or even how it worked. But, I got the parts and wire, wound it and it worked very well. I needed to know why. Thus began my search thru the sparsely published art of transmission line transformer theory and design.

I’ve since amassed a lot of information and have been corresponding with two professional RF Engineers, G8JNJ and VK2OMD, who have greatly aided my understanding of RF transformers. In the upcoming months, I’ll be sharing some of the fun stuff that I’ve learned in antenna types, operation, refinement and more specifically, basic matching transformer design and construction. I’ll include very simple, math to get you in the ballpark; explanations on how they work, what to use for what; why they’re made that way, how to test them to weed out the bad ones; those expensive commercial units with nice stickers that appear to work well but actually don’t and why. I’ll also include reputable sources for the components for those who may be so moved to experiment. 73’s all. Dave AD6AE

Effort to Save Marconi Towers in Canada – Public Invited to Vote on Project

04/10/2025

Article Courtesy Of ARRL

Photo Courtesy Of The SWLing Post

There’s an effort underway to save some of Marconi’s original towers, and an online poll is open for people to vote on it being a restoration project through the “Next Great Save” project from the National Trust for Canada.

Some of Marconi’s first messages were received and transmitted using the Battle Harbour Marconi Towers, thought to be the last of their kind standing in North America. News of Admiral Robert Peary’s 1909 North Pole expedition was transmitted by these towers. After 100 years, the twin towers are in need of repair.

To honor 150 years since Marconi’s birth, there are a number of events planned around the world to observe Marconi’s birthday and International Marconi Day.

In the United States, from the Port of Baltimore, Maryland, the Nuclear Ship Savannah Amateur Radio Club will operate K3S on April 26 from 1330 – 2100Z. Check spotting networks for frequency. See QRZ.com info for Savannah Award qrz.com/db/k3s. A QSL card is available by contacting Ulis Fleming, 980 Patuxent Rd, Odenton, MD 21113.

The Great South Bay Amateur Radio Club in Babylon, New York, will operate W2GSB from the Babylon Village Historical Society Museum for Marconi Day on April 26, 1300 – 2030Z. Frequencies include 28.340, 21.250, 14.246, and 7.245 MHZ.

Photo Gallery

Click on the Links for each state information http://nebraskaqsoparty.com http://miqp.org http://va3cco.com http://quebecqsoparty.org

List http://www.newmexicoqsoparty.org, http://www.gaqsoparty.com, http://www.radioclub-carc.com

Click on the Link below for more information on the convention

Convention LINK http://dxconvention.com