increasing lcl antenna tuning bandwidth 779MHz-928 MHz - Page 1
Forum section: electronics / projects

author: electronx
author_rank: Regular Contributor
timestamp: December 08, 2024, 12:47:09 pm
content: I want to tune a helical antenna to a bandwidth between 779-928 MHz, the center frequency is 853.5 Mhz. Before this, I set up an lcl tune circuit and observe the s11 parameters. S11 reflections are quite good in narrow band, but the bandwidth is quite narrow. What should I do to expand the bandwidth at 779-928 MHz?

author: jwet
author_rank: Frequent Contributor
timestamp: December 09, 2024, 12:00:19 am
content: Not what you want to hear, but you can't really. Antennas, especially small ones are not broad band.You COULD make an antenna tuner with a bank of L's and C's like Hams do on HF. It wont be easy at these frequencies though.Do you need "continuous" coverage over the range or just a couple spots? A dual band matching network or a diplexer is possible but a good low loss broadband match kind of defies physics. If you succeed, let me know, it's a holy grail.Good Luck.

author: electronx
author_rank: Regular Contributor
timestamp: December 09, 2024, 07:24:02 am
content: yes, the wavelength for 729MHz is approximately 41 cm. lambda divided by 4 is at least 10 cm. the antenna I use is 1.8 cm, everything looks interesting up to this point, but there is a situation. On the PCB try to I remade, the guys managed to catch the same frequency range with the same antenna, the same tuning network. This is really interesting. I just don't have access to stackup

author: ftg
author_rank: Frequent Contributor
timestamp: December 09, 2024, 01:01:34 pm
content: Quote from: jwet on December 09, 2024, 12:00:19 amInfineon, pSemi and muRata among others do offer "Digitally Tunable Capacitors" for use as antenna tuners on those frequencies.But using those requires also implementing a return loss bridge, measurements and the code for finding the optimal match.As for a broadband match, one can always make the Q worse and add a lossy element like a series resistor to broaden the S11 bandwidth.Low loss and wide band don't happen in a small space.

author: uer166
author_rank: Super Contributor
timestamp: December 09, 2024, 06:47:41 pm
content: Quote from: ftg on December 09, 2024, 01:01:34 pmThis isn't gonna improve the energy radiated though would it? S11/S12 parameters don't tell you much about actual antenna performance. If it's a resonant antenna element as it usually is, I don't know how you'd be able to appreciably increase its' bandwidth if the geometry is fixed. Normally you'd use a different antenna geometry to actually increase bandwidth. E.g. a dipole with thin wires is very narrow band, thick tubes more wideband, and finally cone-shaped elements even more wideband still. A multiband antenna is also an option with multiple resonating elements.

author: jwet
author_rank: Frequent Contributor
timestamp: December 09, 2024, 07:49:02 pm
content: This is kind of like old hams trying to get their antenna swr to exactly 1. A 50 ohm dummy load has a great swr too.Their are somewhat broader band structures that are usually made with tapered or spiraled striplines on exotic substrates. It usually involves a high wizard that goes off with a $100k piece of Field Solver software for a week and hands you a bill for $50k. Its in the literature for military radar but it mostly at much high frequencies.

author: uer166
author_rank: Super Contributor
timestamp: December 10, 2024, 01:47:31 am
content: Quote from: jwet on December 09, 2024, 07:49:02 pmIt's all applicable for lower frequencies too though!OP, sounds like you need to look at LTE antennas that are already multiband, your range falls exactly into the standard bands, e.g:https://www.digikey.com/en/products/detail/taoglas-limited/PCS-55-A/21704203

author: ftg
author_rank: Frequent Contributor
timestamp: December 10, 2024, 11:48:28 am
content: Quote from: uer166 on December 09, 2024, 06:47:41 pmNot at all, but it can make the transmitter happier and sometimes enable them to work in situations where the internal protections would not allow them to.

author: jwet
author_rank: Frequent Contributor
timestamp: December 18, 2024, 07:57:37 pm
content: Forgot to mention, there is a decent book called "Small Antenna Design" by Doug Miron. Pretty rare topic. Maxim used to make UHF transmitters for key fobs and this was the bible.

author: freetoshare
author_rank: Newbie
timestamp: March 04, 2025, 02:36:22 pm
content: I can dive into it and find a solution if you can share your model, the S11 plot doesn't tell too much information to provide the proposal.Please keep in mind that antenna performance is related surrounding environment, application and PCB ground etc, that is too much matters need to be considered to create a promising antenna.