Portable Flag Antenna for MF/HF Radio Direction Finding
Portable Flag (Terminated Loop) Antenna For MF/HF Radio Direction Finding
Basic details on the portable flag antenna I designed for tracking down RFI (radio frequency interference) on MF (medium frequencies) and HF (high frequencies), as well as some tips and tricks on its use.
Portable Flag Antenna Basic features
- Cardioid pattern which is unidirectional.
- Designed to easily fit in the backseat of my car.
- Typical front to back ratio 12 to 21 dB (2 to 3.5 S units) which is more than adequate for direction finding.
- Designed for 50 ohm systems (SWR measured 1.5 to 1 or less all the way up to 30 MHz when using wire wrap wire for the transformer).
- Normally requires preamp (see preamp comment in sketch below as well as in text that follows below)
Design that I have been using in the field shown below (very basic build).
Antenna wire dimensions
- 24 inches tall
- 48 inches long
Termination Resistor: 680 ohms
Matching Transformer: BN73-202, primary 3 turns, secondary 12 turns
( Transformer wound with 30 gauge wire wrap wire loss test results = 0.18 dB @ 500 KHz, 0.59 dB @ 30 MHz)
Feedline: RG-174A/U (10 feet) plus 3 feet for jumper between preamp and receiver.
- W7IUV (broadband preamp with gain of 20 dB)
- W1FB variable preamp (160 and 80 meter preamp with max gain in excess of 50+ dB)
- KD9SV VLN variable preamp (160 and 80 meter preamp with max gain of 40 dB)
Advanced Receiver Research has a nice line of MF/HF preamps that look interesting such as their model P0.1-30/20VD or their model P0.5-30/20VD. Both models provide 20 dB of gain but the first model goes down to 100 KHz and costs slightly more than the second model which goes down to 500 KHz.
Note: I just use a 9 volt battery to power the W1FB and W7IUV preamp when out in the field.
Portable Terminated Flag shown below with and without 46 inch long extension
Note: while out DFing on foot I have found the 46 inch extension is not really needed, and no longer typically use the extension (use it as shown below). I sometimes still use the 46 inch extension if doing stationary experiments, etc.
Below is a graph showing front to back ratio vs. elevation angle on 160 meters, 30 meters, and 10 meters.
Below are the 160 meter plots showing the vertical plot and then the horizontal plots at 1, 20, 33, and 45 degree elevations.
Below are the 10 meter plots showing the vertical plot and then the horizontal plots at 1, 20, 33, and 45 degree elevations.
Portable Flag Antenna – Tips and Frequently Asked Questions
Question: Where should attenuators be placed when needed?
Answer: When using a preamp with the portable flag, the use of any attenuators should come after the preamp so the signal to noise ratio is not degraded. The portable flag has negative gain so the signal reaching the preamp is small and placing attenuation between the portable flag and the input of the preamp causes the noise figure of the preamp to become a dominate factor which degrades the signal to noise ratio. To avoid signal to noise degradation use attenuators on the output side of the preamp.
Question: I sometimes notice a slight skewing of the pattern in which the null and the peak of the antenna do not yield the exact same signal direction.
Answer: skewing of the pattern can happen if you are located close to other metallic objects that can reflect or re-radiate the signal or cause part of the antenna to couple to the metallic object. Always make sure you use the portable flag away from other objects to avoid pattern skewing and this is typical advice when using any kind of direction finding antenna. Use the portable flag when out in the clear for best results.
Question: I used enamel coated wire instead of wire wrap wire when winding the transformer and notice that my maximum SWR is closer to 2.0 to 1 on the higher frequencies versus 1.5 to 1 as mentioned in the article.
Answer: This is typical when using enamel coated wire for the transformer in place of the wire wrap wire in this application. Since the run of coax is so short the slightly higher SWR is of no significance as it causes very little extra feed line loss and it does not degrade the performance of the portable flag.
Question: How do I connect the transformer?
Answer: The transformer secondary has 12 turns and it connects to the antenna, whereas the transformer primary has 3 turns and it connects to the coax.
Question: “For the binocular cores, do you count one pass through one hole as one turn?”
Answer: No, one complete turn on the binocular core requires the wire to pass through both holes in the core one time as shown in the picture below.
P.S. thanks to Earl (K6SE) who is now a SK for originally introducing large stationary Pennants and Flags to the ham radio community.
Antenna designed by Don Kirk (wd8dsb)
View NoiseLoop Antenna Review, article on this antenna is published in the March 2021 issue of QST Magazine