Vertical antennas can be very effective in long range communication due to low take off angle of electromagnet waves (for reception and transmission), can be installed in a small area for stealth purposes, yet they can also be hard to match and may exhibit higher noise levels than horizontally polarized antennas.
Fortunately Palomar Engineers has simple building block solutions to solve the matching and noise issues as well as a seldom used vertical design that has a higher radiation efficiency and stronger reception of HF band (1-30 MHz) signals for amateur and commercial radio communication.
Here is a typical vertical antenna system:
Antenna System Components
The typical antenna (vertical or horizontal polarized) has 6 components.
The antenna or radiating element (See Antenna Types below for specific characteristics).
The antenna impedance matching components (BOX “Z” above) to match the antenna impedance to the coax line impedance (usually 50 ohms). For vertical antennas with a feed point impedance less than 50 ohms, you will need a sub-ratio impedance transformer (e.g. 25 ohm load to 50 ohm coax, also know as a 1:2 unun). For vertical antennas with an impedance greater than 50 ohms you will need a step up impedance transformer (e.g. 100 ohm load to 50 ohm coax will require a 2:1 unun). Impedance transformer information is HERE
The Feed line Choke (BOX “FC” above) to suppress the common mode current on the outside of the coax feed line. The Feed line (FC) acts as a stop sign for RF current desiring to flow back on the outside of the coax. The higher the choking resistance of the fed line choke, the less the coax braid RFI common mode current and more power into the antenna system. Feed Line choke alternatives are HERE. See ANTENNA TYPES below for specific types of chokes for each antenna.
The Line Isolator is used to bleed off antenna radiation induced common mode current on the coax braid on long (> 1/2 wavelength) coax feed lines. Note that radio stations with multiple antennas and multiple feed lines, the induced common mode current on a particular feed line may be from an antenna connected to a different feed line! This cross coupling of common mode current is a particular problem in multi-band stations operating at the same time. Line isolators can often reduce this common mode current (which is heard as broad band noise on the receiver) to acceptable levels. Consider the use of line isolators for any coax feed lines that are over 1/2 wavelength at the highest operating frequency used at the radio station. For very long coax feed lines, break up the feed line with multiple line isolators every 1/2 wavelength. Line Isolators (also called TUBE feed line chokes) are HERE.
Coax noise filters (BOX “NF” above) are very useful for all types of antennas but a very effective for verticals since much broadband noise is vertically polarized. Common mode current induced by radiated sources (plasma TV, routers, computers, transmitters, radiating antennas, etc) can be picked up by the outside of the coax braid from the antenna feed point back to the receiver. This portion of the coax braid acts like a receiving antenna, picking up common mode noise signals that override weak signals making them difficult or impossible to hear! The solution? A common mode noise filter with high choking impedance at the receiver end of the coax. Common mode noise filters are HERE.
The radio station is also a key component of the antenna system and has two functions: transmit and receive. Matching the transmitter to the coax feed line is often done with an antenna tuner and receiver systems should be installed to maximize signal to noise ratio. Reducing receiver noise is critical for weak signal reception and the use of coax noise filters AND receiver power supply lines (AC or DC) noise filters is usually needed for optimum reception. Reducing RFI generated by the radio station (you are the SOURCE of RFI) or received by your radio station (you are the VICTIM of RFI) is an important aspect of radio station operations. Palomar Engineers has many solutions for RFI problems – Click HERE to develop alternative strategies depending on your particular situation.
Vertical antennas are usually classified with regard to their length in resonant wavelengths. For example: 1/4 wavelength, 1/2 wavelength, 5/8 wavelength, etc. In general you would choose a vertical with the highest feed point impedance (radiation resistance + ground loss resistance) so as to keep radiation resistance as high as possible compared to ground loss resistance which may be as high as 20-30 ohms otherwise much of your power delivered to the antenna feed point may be diverted to the ground resistance (which keeps the worms warm) rather than to the radiation resistance of the antenna (which radiates the signal).
1/4 wavelength vertical
The popular 1/4 vertical antenna is essentially 1/2 of a dipole (often called a unipole) with the other “half” of the antenna a good grounding or counterpoise (raised above physical ground) system. The typical resonant feed point impedance of a 1/4 wavelength antenna (with a reasonable ground system of at least 4 raised radials or 8 radials at ground) is 30-40 ohms. This antenna may be feed directly with a 1:1 coax feed line choke (to direct the RF flowing on the inside coax braid directly to the ground system and prevent it from flowing back on the coax and radiating RFI). Due to the slight impedance mismatch (30 ohms to 50 ohm coax), a small SWR will exist, and depending on the length of cable and frequency of operation and coax line loss at the operating frequency, the coax power loss may or may not be of concern. Usually high SWR is of concern at higher frequencies (over 15 MHz) since coax tends to have higher losses at higher frequencies. For purists who want to minimize SWR and transfer the maximum to the antenna we have 1:1.5 ununs for 36 feed point to 48 ohm coax transformations)
1/2 wavelength vertical
The typical resonant feed point impedance of a 1/2 wavelength antenna (with a reasonable ground system of at least 1 raised radials or 2 radials at ground) is 2,000-4,000 ohms and can be impedance matched with an LC circuit or high ratio impedance matcher such as the Bullet-64 or 64:1 unun which matches 3200 ohms to 50 ohms up to 500 watts PEP.
The typical resonant feed point impedance of a folded unipole (1/2 of a typical folded dipole using 300 or 450 ohm ladder line which is shorted at the top end opposite of the feed point) with a reasonable ground system of at least 4 raised radials or 8 radials at ground) is 150 ohms.