# Trying to identify and understand this antenna design - ground independent dual band vertical

This is the insides of a common dual band antenna 2m/70cm, made by diamond or nagoya. its generally labelled as: 1/2 wave radialless (144MHz), 2 x 5/8 wave radialless (430MHz) Its meant to be ground independant.

Im trying to find the design name for the tuning/matching circuit at the bottom and how it works.

• For example what stops the signal going down to ground at L1 rater than going up the elements ?
• Does the coil below the C1 tap point provide the counterpoise/ground for the antenna, and the part of coil above act as matching/loading coil for lower element ?
• calculators/plans around for this type or antenna ?
• generally how does it work, im also trying to model it such as in 4nec2

simulate this circuit – Schematic created using CircuitLab

• Inductors will need to be modelled as parallel LC circuits, they will be designed to self-resonate somewhere near the bands of interest. Feb 3, 2021 at 7:27
• Where is the feedpoint? Diagram says the "metal base cylinder" is connected to ground, but doesn't show the connection; are you sure they are shorted together? Feb 3, 2021 at 8:36
• Also keep in mind that DC connectivity to ground does not mean RF connectivity to ground. Consider a simple loop antenna: at DC the entire thing is a short circuit, and yet if the loop is made one wavelength in circumference it can be a very effective antenna. Feb 3, 2021 at 14:38
• @BrianK1LI yes, the base that screws on to pl259 is a metal tube connected to gnd/shield, inside tube is the coil and cap. the antenna elements are then connected to the top of the tube which is plastic capped and has active come out of centre. Feb 4, 2021 at 22:30
• Background: The radiation resistance of such spiral-wound antennas (alone) is related to its physical length (end-end), which for a Nagoya NA771 is ~15-1/4" -- about 0.39 lambda at 146 MHz Coil L3 shown on your drawing is an attempt to zero out the reactance of that end-fed radiator, but does little to increase its radiation resistance. The network at the base of the radiator provides an ~tolerable match of the input Z of the antenna system to what the transmitter expects to see as a load: probably 50 ohms. Feb 8, 2021 at 7:49

This arrangement:

simulate this circuit – Schematic created using CircuitLab

is called an L-match.

Adding a tap to the inductor creates an autotransformer which allows an additional degree of freedom in the tuning network. It's useful here because the impedance of the half-wave element is much too high; the autotransformer steps it down to something closer to 50 ohms, reducing the mismatch that must be corrected by the L-match.

This is very similar to an equivalent circuit for a gamma match, the only difference being the autotransformer is flipped around.

• The L match calcs and circuit examples i find online have the cap tied to ground, not with the source in series. Feb 15, 2021 at 1:57
• @HaydenThring It's named an L-match because the two components make an L-shape. It's still an L-match with whatever components you put in there, and you can flip them around depending on if you want to step or down, or if you want a high- or low-pass characteristic. Feb 16, 2021 at 21:46
• Ok, thanks for your help. With how my diagram has the capacitor connecting part way along the inductor (as a tap?) would i model that by adding another inductor in series after the l match to account for the part of the inductor above the tap point ?? Feb 16, 2021 at 22:16
• @HaydenThring The tapped inductor is an autotransformer. You can't model it as a separate series inductor since that wouldn't model the shared magnetic flux. How you model it depends on what kind of model you are making. Feb 18, 2021 at 15:32
• @HaydenThring I don't know anything about 4nec2, maybe try asking a new question along the lines of "how to model an autotransformer in 4nec2". I'm sure other people here know. Feb 18, 2021 at 23:11

The matching circuit looks similar to the type of N:1 balun/unun used to feed an end-fed half-wave without too great an impedance mismatch. Except in this case L3 shortens the dipole at its center, and the portion below half of L3 is the counter-pose to the quarter wave section above half of L3.

• Oh, so i was maybe thinking about it wrong, so do the 2x 5/8 'halves' act as a 70cm dipole, not 2 individual stacked elements (collinear?) but can you have a dipole fed from the end ? is it then just a end fed and not a dipole... Id also assumed which i think is still correct that both elements act as one to form a 1/2 for 2m. Feb 4, 2021 at 21:48