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I'm working on the design of a dual band transmitter for use on a high altitude balloon. I'll be transmitting on 2 meters and 20 meters (APRS and WSPR) with about 50 mW. To keep the weight to an absolute minimum, I'm designing it to use a single antenna for both bands. Using 4nec2 modeling I came up with an off center fed (20M 1/2 wave) dipole having a feedpoint positioned where the impedance is about 200 ohms on both bands (using 38 AWG wire.) A diplexer circuit will merge the outputs of the 2M and 20M RF paths and match the impedance to the antenna. I estimate this antenna configuration should weigh less than 1 gram. Any advice from experienced wire antenna/ high altitude balloon experimenters before I commit too much to hardware?

whalphen K8VFO
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It is equally important to consider the pattern of the antenna. If you hang a 1/2 wave antenna vertically, there will be minimal radiation in the earth facing direction. This will be most problematic for HF NVIS contacts.

You may also wish to check the RF resistance of 38 AWG wire on 2 meters. I roughly calculate it to be > 100 ohms at 146 MHz for 10 meters in length. This will have significant effect on the efficiency, and thus the gain, of the antenna on 2 meters. At 20 meters, it is > 40 ohms, which can also be a significant factor in the performance of the antenna.

Have you modeled the directivity and efficiency of the antenna to ensure it meets your path budget?

Glenn W9IQ
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    "Have you modeled the directivity and efficiency of the antenna...?" Good suggestion. _And that's easier to do than one might think_. I use EZNEC. There are other antenna modeling programs. – Mike Waters Jan 27 '18 at 02:42
  • "If you hang a 1/2 wave antenna vertically, there will be minimal radiation in the earth facing direction". That's only true when the receiver is directly under the balloon, isn't it? – Mike Waters Jan 27 '18 at 02:45
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    I did check the directivity in 4nec2 modeling software. The OCF configuration gives reasonable lobes at a downward angle at 2 meters. (At 2 meters, I have used vertical dipoles on multiple launches with no problem.) This will be a long distance floater. For HF, I prefer having the radiation pattern out to the sides for longer distance propagation while over the oceans, etc. I'll rely on others to give reception reports via WSPRnet. – whalphen K8VFO Jan 27 '18 at 03:05
  • As for the RF resistance, I've factored that into the matching calculations. Will also take another look to be sure I understand the impact on efficiency. – whalphen K8VFO Jan 27 '18 at 03:08
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    @whalphenk8vfo For the HF calculations, you may find it helpful to run FRIIS calculations for stations that are LOS to the balloon. What is your flight ceiling? – Glenn W9IQ Jan 27 '18 at 03:12
  • The radiation resistance of the 20 meter dipole will be 72 ohms. A 40 ohm RF resistance will make the antenna ~64% efficient or ~0.2 dBi gain. – Glenn W9IQ Jan 27 '18 at 03:20
  • A quick re-run of the modeling shows 73% efficiency at 2M and 79% at 20M. These should both be well within my budget -- unless the actual hardware tests give me lower than expected output from the transmitters. Interesting how there is so much interdependancy among the system components. Change the wire diameter and I have to change the impedance matching which means I have to rerun my diplexer filter designs, etc. Increasing the wire diameter may improve my path performance, but may blow my weight budget... – whalphen K8VFO Jan 27 '18 at 03:23
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    The joys of engineering... Have fun with the project! – Glenn W9IQ Jan 27 '18 at 03:25
  • Flight ceiling is expected to be between 35000 and 45000 feet. I haven't done any FRIIS calculations, but will. Thanks for the advice. I'm not too concerned based on experience others have had. But it won't hurt to check. I'm expecting an output to the antenna of about 30 to 50 mW on each band. I've had success with 50 mW on 2M in the past. Others have been successful with 10 mW. On 20M others are having success with less than 15 mW output. The WSPR mode needs very little. I've had many trans-oceanic contacts using 250mW into a dipole stretched thru the trees in my back yard. – whalphen K8VFO Jan 27 '18 at 03:34
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    Is that 40 ohms end-to-end, or an apparent 40 ohms at the feedpoint of resistive loss? – Phil Frost - W8II Jan 28 '18 at 00:01
  • Re Radiation Patterns - your 20 m pattern will be fine, dipole-like. Anyway, you're planning on it skipping to distant stations. But your 2 m pattern might be a mess, from a long wire, and on 2 m I'm sure you are expecting line of sight communication to the launch site. Check this pattern. You might need a trap or two in the long wire, these could be an SMD inductor and capacitor, so no significant weight. – tomnexus Jan 28 '18 at 06:34
  • You are right. 2M will be for short distance APRS. 20M will be for ionospheric skip to distant receivers. I did check the 2M pattern using 4nec2 modelling software. Got lucky. Because it's an off center fed configuration, the distant field shows a nice 7 dBi lobe at about 155 degrees down from vertical and many small 0 dBi lobes from 90 degrees down to 135 degrees. – whalphen K8VFO Jan 28 '18 at 13:57