# How much loss for 1 to 1.5 inch Coil (rubber ducky) Antenna at 70cm band?

1What is the likely antenna loss for this (coil hidden inside rubber ducky)

1 to 1.5 inches coil antenna, when placed vertically over 4 to 6 inches each side square ground plane

as compare with a proper 0.25 wavelength wrip antenna placed over ground plane 0.5 wavelength square?

Without access to network analyzer, what tools and how one can calculate the impedance? How to calculate matching network to match it to 50 ohm transmitter?

I refers to these type of coil antenna as in lower right of this photo as originally designed for walkie talkie. 70cm band is about 10% off the walkie talkie freq. Presumably, one can start by cutting a few percent length and start tuning from there.

EDIT: Here is dimension and photo of actual antenna on hand. The long 'end' (end of 8.5mm segment) is the feeding point to be soldered to PCB. The end of 28.0mm segment is not-connected. The antenna coil will be 'vertically mounted' on copper pcb acting as ground plane. The ground plane will only be 100mm, smaller than the ideal 0.25 wavelength, due to size restriction.

EDIT2: @K7PEH Feed point is the end of the straight-segment (leftmost in photo). Wire diameter 0.58mm. Coil diameter is 4.23mm. Coil 27 turns. Total wire length 359mm. Appreciate if you may enter data, save screen dump step-by-step in a words file and email me the work flow file so that I can learn. Specifically, how is the antenna specification entered as 'segments' into the program.

• edited add walkiee talkie photo showing coil antenna – EEd Aug 22 '14 at 7:17
• You want to measure antenna gain (or loss), or feed impedance? They are not the same thing. Also, what equipment do you have? – Phil Frost - W8II Aug 22 '14 at 18:38
• There are 3 questions. 1. For ant gain, I tend to think the antenna loss may be in region of a few to 10 dB. Just guess and hope someone may comment, either with a accurate number or 'real life test result and/or feeling'. 2. For imp., understand proper method method is net analyzer but no access now. Is there a calculation method, even if not 100% accurate, still give a range. 3. After having imp. from step 2, how to calculate match network to transform the ant to 50 ohm pure resistance? – EEd Aug 22 '14 at 18:50
• As for equip., only SWR meter, hand held transmitter and dummy load. Times before, old work place has proper RF lab with all needed equipment. Not now. So, back to 'ham level' of using simple gear, calculator and try to get something within a range, not 100% accurate. – EEd Aug 22 '14 at 18:56
• If you have three questions, it's better to ask three questions. – Phil Frost - W8II Aug 22 '14 at 23:53

Answer of sorts (or, not an answer at all) to questions about rubber ducky antenna. This follows the comments dialog above as this post is too large for a mere comment.

I volunteered to run the NEC2 calculation for this antenna and after the OP provided more details I started doing back of the envelop computations since I am away from my main computer for a few days (where I have NEC2/NEC4 and my Mathematica solution modules). I did a simple reactance calculation of the coil itself at the designated frequency of 462 MHz which is open for FRS/GMRS operations that I think is associated with the rubber ducky antenna of the HT.

The inductance is approximately $0.43\,\mu H$ and the resulting inductive reactance $X_L$ is: $$X_L = 2\,\pi \, f\, L = 2\times 3.14\times 462\times0.43 = 1248\, ohms\qquad \text{(rounded)}$$ This rather large impedance means that this rubber-ducky (for it to work at all) has high capacitive coupling to the HT and even probably to the person holding it which naturally form part of the antenna system.

Therefore, I posted some questions on NEC2 distribution list and one answer (from the creator of EZNEC, who is a Guru/Wizard with NEC2 and NEC4) provided these comments (quoted directly from his e-mail):

It's not practical to model with NEC-2 for several reasons:

1. The "ducky" is only a part of the antenna system. At least as important, and likely more important, is the person holding the radio which can have a major impact on the pattern. Whatever the current flowing into the base of the "ducky", an equal amount of current flows along the outside of the radio and the person holding it. This current causes radiation just as the current on the "ducky" does, but from a much larger "antenna".

2. My experience is that modeling an inductively loaded antenna as a wire with one or more ideal inductive loads doesn't provide accurate results. I believe this is because the current changes significantly from one end of the actual loading inductor to the other, while currents are the same at the ends of idealized loads. Even using multiple loads doesn't do as well as modeling the actual inductor as a helix.

3. NEC-2 requires that wires be spaced at least several diameters apart. A typical "ducky" helix has turns spacing closer than this, making modeling the inductor portion of the antenna as a helix questionable.

4. NEC-2 doesn't take into account proximity effect, which might be significant with closely spaced helix turns. Presuming you're trying to evaluate the efficiency of the "ducky", this means that the efficiency shown by the model will be overly optimistic even if you use realistic values for wire resistivity.

5. NEC-2 has no way to account for the dielectric surrounding a "ducky". This will alter the resonance of the antenna in a way not predicted by the program.

Sorry, there are just some things that can't be practically modeled with NEC-2. This is one of them.

End of Quote.

So, sorry to say that when I get back home I will probably not try to model this antenna as I volunteered to do. Hopefully though, there is sufficient content in this answer to give the OP more information to make judgements or decisions regarding the rubber-ducky antenna.