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DESIGNING ANTENNAS & OTHER THOUGHTS

For the past year or so, I have been receiving e-mail asking me a variety of questions regarding amateur radio and old time radio. Most of the questions, I have tried to answer and I apologize to anyone I may have missed. Sometimes it seems e-mail does get lost or I misplace it on the hard drive.

RCA World's Fair model, Chicago

Picture taken at Historic Radio Museum in Ligonier, Indiana using a Sony Mavica 7.

For those interested in old time radio, a worthwhile visit is to the Antique Radio Museum located in Ligonier IN. This is in northeast Indiana not too far south of the I80-90 toll road. There are over 400 antique radios on display there. Also on display is a collection of novelty radios. Currently the hours might be 10 a.m. to 3 p.m. Tuesday, Wednesday, and Saturday. If you are planning a visit check www.ligtel.com or 1-888-417-3562 and confirm the days and times. The museum is staffed with volunteers and when we were there we were told that if they had more volunteers, they would be open additional days.

radios in model ships

Picture taken at Historic Radio Museum in Ligonier, Indiana using a Sony Mavica 7.

Much of the e-mail I receive has to do with antennas and a number of people have asked me what kind of wire antenna they should use. This is kind of hard to do because it will depend on a persons operating preferences and space limitations if any.

To clarify this, I will go through the process of how I would design an antenna to meet my needs. I would like to operate on as many of the HF bands as I can and though I prefer to use CW, I would like to have an antenna that permits phone operation as well. Since I live in the city, an antenna for 160 meter operation may not be too practical but we'll get to that later. With a 50' by 125' lot I want to be able to put up some kind of antenna that will work on the 80-40-30-20-17-15-12 and 10 meter bands.

The antenna requirements here are far different than that of a commercial station which is licensed to operate on a single frequency or perhaps several discrete frequencies. A resonant antenna system can be fairly easily designed to meet these requirements.

many old radios

On the other hand, I want to be able to operate anywhere in the above eight amateur bands and some of these are relatively large. Eighty meters for example is one-half Mhz wide and 10 meters covers almost 2 Mhz. The total bandwidth of the ham bands 80 through 10 meters if one adds up the various segments is 3.55 Mhz. Compare these with the AM broadcast band which is a little over 1 Mhz wide. I want to be able to operate anywhere in these two widest HF amateur bands as well as the six others. It will be pretty difficult to do this with a resonant antenna system.

Crosley Pups

Picture taken at Historic Radio Museum in Ligonier, Indiana using a Sony Mavica 7.

Don't get me wrong, a resonant antenna system is fine for some purposes, but it just doesn't suit my requirements. If on the other hand I wanted an antenna to use for operation on the Indiana Phone nets (3.910Mhz), a half wave dipole cut for that frequency fed with 50 ohm coax direct to the transceiver would be a great choice as long as I didn't want to stray very far from 3.910Mhz. A resonant half-wave dipole has an impedance generally in the range of 50-70 ohms near the resonant frequency thus presenting a pretty good match for 50 ohm coax and the fixed 50 ohm output impedance of most modern rigs.

As one tries to operate at frequencies differing from the resonant frequency however, the impedance changes and a mismatch begins to develop thus resulting in a higher SWR. The bandwidth over which the SWR is 2 to 1 or less with a resonant wire antenna is generally about 2% of the center frequency. Therefore a 75-80Khz bandwidth could be expected for an antenna designed for 3.910 Mhz. A 40 meter antenna would have a bandwidth for 2 to 1 SWR of about 150 Khz and so on.

If I try to use a resonant wire antenna to meet my requirements of operating anywhere over the 3.55Mhz of the eight HF bands, I will probably need to have a number of individual antennas...one for each of the eight bands and several for 80 meters. I could probably cut this down a little since a resonant half-wave dipole will present a pretty decent match on the third multiple of the frequency for which it was cut. Thus a 40 meter antenna will usually work fairly well on 15 meters and I might get by with an eighty meter antenna cut for the low end of the CW portion on 30 meters. This would still leave me with at least six antennas to erect and I can only be using one at a time.

My purposes are better met with a non-resonant antenna system. This way I can put up only one antenna and use it on whatever frequency I choose to operate. However, I must be willing to use an antenna tuner or transmatch. A multi-band antenna that has worked very well for me and others over the years is a variation of the G5RV dipole. This antenna is 104' long (52' each side of the center insulator) and fed at the center with balanced feed line (this can be 450 ohm ladder line, home made open wire feeders, or 300 ohm TV twinlead for output power of 100-200 watts or less). You may want to read my previous column on antennas for a longer discussion of this kind of antenna. A drawing of this antenna follows with O representing the center and end insulators.
                              52 ft                      52 ft
O_____________________O____________________O
                                                 | |
                                                 | |
                                                 | |
                                                 | |
                                                 | |            ladder line or twinlead
                                                 | |
                                                 | |

This antenna will also work on 160 meters though it probably won't make you a DX contender on the band. Simply tie both wires of the feedline together and attach them to the single wire output of the tuner. A good ground is important to get decent results on 160.| |

Any antenna is a compromise and this one is certainly no exception but it does offer the versatility of multi-band operation with one antenna and it doesn't take up a large amount of real estate. This antenna can also be erected as an inverted vee which saves some additional space and requires only one support. Some disadvantages are that balanced feedline is more difficult to route into the shack than coax, feedline connections are more prone to breaking at the antenna connection than coax (I inspect mine a couple of times a year though it is fairly easy to tell there is a problem from how the tuner behaves), and some people would consider the need for a tuner a disadvantage.

Perhaps these thoughts will help someone in choosing among the many antenna possibilities.

first radio station

Picture taken at Historic Radio Museum in Ligonier, Indiana using a Sony Mavica 7.


73's...W9PPG

article ©1999 Wm. J. Weinhardt

W9PPG, Bill Weinhardt's Radio Columns
1. Why use CW?
2. Learning Morse Code
(the easier way)
3. Great Fun
with CW

4. Antenna Ideas
5. Old Equipment
I've bought and made
6. More Radio Nostalgia
7. W9ASX, My Elmer, Glen Rogers
8. Surplus Equipment
9.Questions from my e-mail, with answers
10. Antenna design; then Indiana's Historic Radio Museum
Science Fair Project, Keyer
Who saved my life?
All columns ©copyright Bill Weinhardt 1996-1999


Email W9PPG at wswart@parlorcity.com

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