# Input impedance of a shortened dipole antenna

I want to make myself a antenna for 40m and 20m wavelength band, but i don't have enough space for a full scale dipole, and even for more compact configurations like delta-loop or inverted v, so i want to create a shortened dipole in configuration of a whole-length solenoid with small (compared to wavelength) diameter. It should work the same as a wire in terms of transmitting energy (J. S. Belrose article, QST, Sep 1953) but with increased inductance throughout whole length of it.

And if i recall correctly, that for a shortened dipole it's input impedance starts to have a reactive capacitive component which needs to be compensated with a "lengthening inductance".

So the question is, how much inductance i need to have to appropriately compensate for a shortened regular dipole? And what the active resistance will be so i can match the antenna? I'm having trouble finding the formulas for overall input impedance.

Thanks in advance.

• @webmarc actually it is not, i was browsing and i want to make a whole solenoid antenna from J. S. Belrose article, QST, Sep 1953. He explains relation of effectiveness of radiation to distribution of current in antenna like this: (imgur.com/a/wnFlJLV). According to this article a small enough solenoid works just like a wire with increased inductance, and a whole-length could would be a bit more efficient than a coil in the middle (also structurally more stiff, with no overhangs). And also, that calculator from answer is for monopole in ground-plane configuration. Feb 19 at 23:02
• Although i got my answer from question: What is the impedance of a 1.25 λ dipole antenna? Feb 19 at 23:21
• @ZecosMAX, how much horizontal space do you have? How many points of support do you have, and what are the heights of those suspension points? Feb 20 at 15:25
• @LouisSeaman why would that matter? Feb 20 at 18:35
• Those questions matter because a) horizontal space will define the overall length of the antenna, which will determine where your coils wind up; and b) your height above ground will be an influence on impedance, which you may or may not have to compensate for. Mar 2 at 18:46

## 1 Answer

ZecosMAX: The calculation that doesn't really tell you specifically what you need to do for a dual-band antenna can simply be found here: https://www.66pacific.com/calculators/coil-shortened-dipole-antenna-calculator.aspx The issue is using one dipole antenna for two bands, where the input may be a current loop on one band, and a current node on the other, if not designed specifically for what you are trying to do. If you have an antenna tuner, you can make your radio happy, but that may not make for an efficient antenna on one or both bands. Also, keep in mind that inductors narrow bandwidth. Finally, I would make some recommendations if you disclosed your dimensional parameters...but that's your choice.

• Actually i haven't considered narrowing of a bandwidth, thanks for mentioning it! I'll try anyway with coiled-dipole just out of curiosity but maybe i'll go for an EH-antenna, if bandwidth won't be wide enough Mar 5 at 2:12
• If you have enough room for a 20M halfwave center-fed dipole (~32 feet) and place your 40M inductors/coils at that point, the coils will act like a 20M trap, and you should have fairly good bandwidth on 20M, but 40 will be narrower. Again, given the room, you could also make an end-fed 1/2 wave (Inverted L if you don't have a 66' tree to hang it from) with a 49:1 UnUn. This way you have a 1/2wave at 40M and a full wave at 20M, Both with an approximate 2450 Ohm feed-point impedance. This is what I use, and I choke the coax at the UnUn and have a 6'8" counterpoise on the coax shield. Mar 6 at 12:21