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broomstick antenna parameters

Mar 13th 2019, 20:29


Joined: Oct 14th 2018, 20:05
Total Topics: 0
Total Posts: 0
Hello, I am seeking help with my first antenna build to go with a superhet radio I will be building.
The radio design is a 1991 book " Shortwave Superhet Receiver construction" by R.A. Penfold. (#276).
This design uses a MOSFET mixer and two IF stages. There are 9 possible add-ons: input filter and RF pre amp, product detector and BFO etc. This gives me plenty of room to experiment and learn.

I am awaiting parts- it took awhile but I've found everything, including the Toko RF transformers. In the meantime I settled on this antenna as being potentially more useful than a long wire, and not having to hire anyone to climb the palm trees at my home. The author calls it the broomstick antenna, a 1.7 m (6 ft) PVC pipe with a 30 cm capacitance hat at the top

The radio will potentially tune 80-10M bands so I want the antenna to be able to do so also.
The author suggests dividing the coil wire into three sections: 1/3 spaced 2 wire diameters apart, 1/3 tightly wound, and 1/3 one wire diameter separation
Using 40 meters total length of 14 awg magnet wire ( 1.7 mm diam) and 33 mm diameter PVC pipe, each section requires 124 turns.
I calculate the inductance of the sections at 29, 85 and 44 uH respectively. The "Q" for each section is very high unless I have made a mistake: well over 600 depending on frequency.
This broomstick design is also from the 1990s. My questions are
1. I am using 40 meters of wire to have a half wave at 80 M. Is this reasonable?
2. The coils only use 120 cm of the 172 cm usable length of the long pole. I assume I should place them next to the top next to the "cap", and not in the middle of the pole.
3. Understanding this is all experimental, would a design with 2 or 4 sections of wire spacing make more sense as a first try?
4. The author mentions having the antenna as close to the receiver as possible. I want it outside on my porch, approx 6m away. I can insert a ground rod close by. Should I use coax for the antenna lead?
5. The author mentions using an antenna tuner within 1m of the antenna. In other words at the base of the antenna. First the radio will have an input filter and RF amplifier before the MOSFET mixer and first IF stage. Doesn't the "input filter" which is an LC resonator built with a variable capacitor and a Toko RF transformer serve the same function as an antenna tuner, while also including the lead in wire?

I was a ham operator long ago at 11 years old, WN2TBG. I had a Hallicrafters receiver and a borrowed 2M transceiver. The hobby was fun but didn't survive the distractions of high school. Recently I started building radios, crystal sets and a regen receiver kit. They work, sort of. Part of the problem perhaps, is that much of my house is plaster and metal lathe which might act like a Faraday cage blocking signals? At any rate the regen kit works best outside, which is why I want to move my antenna outside.
I am progressing thru levels of complexity, and will eventually build something that really works.
Mar 14th 2019, 12:27


Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Ideally, you would attach a high impedance preamplifier to the base of the antenna. This would allow excellent signal transfer even without the expense of a tuner. You may want to study "Active Antennas," originally invented by R W Burhans to better understand this concept.

If you add a length of coax, the capacitance of the coax will shunt a significant amount of signal if the antenna has a high impedance, and a negligible amount when the impedance is low. The need to tune an antenna to resonance may be considered by some users to be an unnecessary hassle. Portable shortwave receivers often come with untuned whip antennas connected directly to a high impedance preamplifier.

Zak Lau W1VT
ARRL Senior Lab Engineer

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